feat: implement Slack data source connector (#15188)

### What problem does this PR solve?

Closes #15187.

RAGFlow shipped a Slack connector
(`common/data_source/slack_connector.py`) but it was never usable:
`Slack._generate()` in the sync worker was a `pass` stub, the
connector's document-generating code was incompatible with the current
data model,
and Slack was commented out of the data-source settings UI. As a result,
teams had no way to index Slack channels/threads into a knowledge base.

This PR completes the connector end to end.

**Backend**

- `common/data_source/slack_connector.py`
- Rewrote `thread_to_doc` to produce a blob-based `Document`
(`extension`/`blob`/`size_bytes`). The previous implementation built the
doc with a `sections=[...]` argument and omitted the now-required
`blob`/`extension`/ `size_bytes` fields, so it raised a validation error
against the current `Document` model. Thread messages are now cleaned
and flattened into a single UTF-8 text blob.
- Added `load_from_state()` / `poll_source(start, end)` generators. The
connector's checkpoint interface is a no-op stub, so both full and
incremental syncs run through a single channel-iterating generator built
on the existing module helpers (`get_channels`, `filter_channels`,
`get_channel_messages`, `_process_message`), with per-channel thread
de-duplication.
- `rag/svr/sync_data_source.py`
- Implemented `Slack._generate()`. Credentials are loaded via
`StaticCredentialsProvider` (the connector requires `slack_bot_token`
and does not support `load_credentials`). Supports full reindex and
incremental polling from `poll_range_start`, plus the optional channel
filter. Modeled on the Confluence/Dropbox wrappers.
- `SlackConnector` was already exported from
`common/data_source/__init__.py`.

**Frontend (`web/`)**

- Enabled the `SLACK` data-source enum and added its form fields (Slack
bot token + optional channel filter), default values, display metadata,
and a Slack icon.
- Added `slackDescription` / `slackBotTokenTip` / `slackChannelsTip`
strings to `en.ts` and `zh.ts`.

**Tests**

- `test/unit_test/data_source/test_slack_connector_unit.py`: unit tests
covering credential loading (`load_credentials` raises,
`set_credentials_provider` initializes clients, missing credentials
raises) and document generation (standalone message + flattened thread,
blob/extension/size_bytes/metadata, and the incremental poll time
window). All 5 pass; `ruff check` is clean.

Required Slack scopes: `channels:read`, `channels:history`,
`users:read`.

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

test/unit_test/data_source/test_slack_connector_unit.py

@@ -0,0 +1,206 @@
+#
+#  Copyright 2026 The InfiniFlow Authors. All Rights Reserved.
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+#
+import importlib.util
+import sys
+from pathlib import Path
+from types import ModuleType
+
+import pytest
+
+
+def _load_slack_connector_module():
+    """Load slack_connector.py in isolation.
+
+    Importing ``common.data_source`` directly would execute its ``__init__``
+    and pull in every connector's (heavy) dependencies. We stub the package and
+    exec only the Slack module, mirroring the Dropbox connector unit test.
+    """
+    repo_root = Path(__file__).resolve().parents[3]
+    package_name = "common.data_source"
+    saved_modules = {
+        name: module
+        for name, module in sys.modules.items()
+        if name == package_name or name.startswith(f"{package_name}.")
+    }
+    package_stub = ModuleType(package_name)
+    package_stub.__path__ = [str(repo_root / "common" / "data_source")]
+    sys.modules[package_name] = package_stub
+
+    try:
+        spec = importlib.util.spec_from_file_location(
+            "_slack_connector_under_test",
+            repo_root / "common" / "data_source" / "slack_connector.py",
+        )
+        module = importlib.util.module_from_spec(spec)
+        spec.loader.exec_module(module)
+        return module
+    finally:
+        for name in list(sys.modules):
+            if name == package_name or name.startswith(f"{package_name}."):
+                if name in saved_modules:
+                    sys.modules[name] = saved_modules[name]
+                else:
+                    sys.modules.pop(name, None)
+
+
+slack_connector = _load_slack_connector_module()
+SlackConnector = slack_connector.SlackConnector
+SlackTextCleaner = slack_connector.SlackTextCleaner
+
+
+class _FakeResponse(dict):
+    """Mimics slack_sdk SlackResponse: dict-like, plus .validate() and .data."""
+
+    def validate(self):
+        return self
+
+    @property
+    def data(self):
+        return self
+
+
+class _FakeSlackClient:
+    def __init__(self):
+        self.token = "xoxb-test"
+        self.joined = []
+        self._users = {
+            "U1": {
+                "real_name": "Alice",
+                "profile": {"display_name": "alice", "first_name": "Alice", "last_name": "A"},
+            },
+            "U2": {"real_name": "Bob", "profile": {"display_name": "bob"}},
+            "U3": {"real_name": "Carol", "profile": {"display_name": "carol"}},
+        }
+
+    def conversations_list(self, cursor=None, limit=None, **kwargs):
+        return _FakeResponse(
+            {
+                "channels": [
+                    {"id": "C1", "name": "general", "is_member": True, "is_private": False},
+                ],
+                "response_metadata": {"next_cursor": ""},
+            }
+        )
+
+    def conversations_history(self, cursor=None, limit=None, channel=None, oldest=None, latest=None, **kwargs):
+        return _FakeResponse(
+            {
+                "messages": [
+                    {"ts": "1.0", "user": "U1", "text": "Hello world"},
+                    {"ts": "2.0", "user": "U2", "text": "Question?", "thread_ts": "2.0"},
+                ],
+                "response_metadata": {"next_cursor": ""},
+            }
+        )
+
+    def conversations_replies(self, cursor=None, limit=None, channel=None, ts=None, **kwargs):
+        return _FakeResponse(
+            {
+                "messages": [
+                    {"ts": "2.0", "user": "U2", "text": "Question?", "thread_ts": "2.0"},
+                    {"ts": "3.0", "user": "U3", "text": "Answer!", "thread_ts": "2.0"},
+                ],
+                "response_metadata": {"next_cursor": ""},
+            }
+        )
+
+    def users_info(self, user=None, **kwargs):
+        return _FakeResponse({"ok": True, "user": self._users.get(user, {})})
+
+
+def _connector_with_fake_client(client, batch_size=10):
+    connector = SlackConnector(batch_size=batch_size)
+    connector.client = client
+    connector.text_cleaner = SlackTextCleaner(client=client)
+    return connector
+
+
+# --- credential loading -----------------------------------------------------
+
+
+@pytest.mark.p2
+def test_load_credentials_is_not_supported():
+    connector = SlackConnector()
+    with pytest.raises(NotImplementedError):
+        connector.load_credentials({"slack_bot_token": "xoxb-abc"})
+
+
+@pytest.mark.p2
+def test_set_credentials_provider_initializes_clients():
+    connector = SlackConnector()
+
+    class _Provider:
+        def get_credentials(self):
+            return {"slack_bot_token": "xoxb-abc"}
+
+    connector.set_credentials_provider(_Provider())
+
+    assert connector.client is not None
+    assert connector.fast_client is not None
+    assert connector.text_cleaner is not None
+
+
+@pytest.mark.p2
+def test_fetch_without_credentials_raises():
+    connector = SlackConnector()
+    with pytest.raises(slack_connector.ConnectorMissingCredentialError):
+        list(connector.load_from_state())
+
+
+# --- document generation ----------------------------------------------------
+
+
+@pytest.mark.p1
+def test_load_from_state_generates_thread_documents():
+    connector = _connector_with_fake_client(_FakeSlackClient())
+
+    batches = list(connector.load_from_state())
+    docs = [doc for batch in batches for doc in batch]
+
+    # Standalone message + one thread (parent + reply collapsed into one doc).
+    assert [doc.id for doc in docs] == ["C1__1.0", "C1__2.0"]
+
+    standalone, thread_doc = docs
+    assert standalone.source == "slack"
+    assert standalone.extension == ".txt"
+    assert standalone.blob == b"Hello world"
+    assert standalone.size_bytes == len(b"Hello world")
+    assert standalone.metadata == {"Channel": "general"}
+    # get_semantic_name() prefers real_name ("Alice") over the display_name.
+    assert standalone.semantic_identifier == "Alice in #general: Hello world"
+
+    # Thread messages are flattened into a single blob, joined by blank lines.
+    assert thread_doc.blob == "Question?\n\nAnswer!".encode("utf-8")
+    assert thread_doc.size_bytes == len(thread_doc.blob)
+    assert thread_doc.semantic_identifier == "Bob in #general: Question?"
+
+
+@pytest.mark.p1
+def test_poll_source_passes_time_window():
+    client = _FakeSlackClient()
+    captured = {}
+
+    def _history(cursor=None, limit=None, channel=None, oldest=None, latest=None, **kwargs):
+        captured["oldest"] = oldest
+        captured["latest"] = latest
+        return _FakeResponse({"messages": [], "response_metadata": {"next_cursor": ""}})
+
+    client.conversations_history = _history
+    connector = _connector_with_fake_client(client)
+
+    list(connector.poll_source(100.0, 200.0))
+
+    assert captured == {"oldest": "100.0", "latest": "200.0"}