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)

rag/svr/sync_data_source.py

@@ -61,6 +61,7 @@ from common.data_source import (
     RDBMSConnector,
     DingTalkAITableConnector,
     RestAPIConnector,
+    SlackConnector,
     SharePointConnector,
 )
 from common.data_source.models import ConnectorFailure, SeafileSyncScope
@@ -999,7 +1000,58 @@ class Slack(SyncBase):
     SOURCE_NAME: str = FileSource.SLACK
 
     async def _generate(self, task: dict):
-        pass
+        from common.data_source.config import DocumentSource
+        from common.data_source.interfaces import StaticCredentialsProvider
+
+        channels_conf = self.conf.get("channels")
+        if isinstance(channels_conf, str):
+            channels = [c.strip() for c in channels_conf.split(",") if c.strip()]
+        elif isinstance(channels_conf, list):
+            channels = [str(c).strip() for c in channels_conf if str(c).strip()]
+        else:
+            channels = None
+
+        raw_batch_size = self.conf.get("batch_size", INDEX_BATCH_SIZE)
+        try:
+            batch_size = int(raw_batch_size)
+        except (TypeError, ValueError):
+            batch_size = INDEX_BATCH_SIZE
+        if batch_size <= 0:
+            batch_size = INDEX_BATCH_SIZE
+
+        self.connector = SlackConnector(
+            channels=channels or None,
+            channel_regex_enabled=bool(self.conf.get("channel_regex_enabled", False)),
+            batch_size=batch_size,
+        )
+
+        credentials = self.conf.get("credentials") or {}
+        if not credentials.get("slack_bot_token"):
+            raise ValueError("Slack connector is missing the bot token credential.")
+
+        credentials_provider = StaticCredentialsProvider(
+            tenant_id=task["tenant_id"],
+            connector_name=DocumentSource.SLACK,
+            credential_json=credentials,
+        )
+        self.connector.set_credentials_provider(credentials_provider)
+        self.connector.validate_connector_settings()
+
+        poll_start = task["poll_range_start"]
+        if task["reindex"] == "1" or not poll_start:
+            document_generator = self.connector.load_from_state()
+            _begin_info = "totally"
+        else:
+            end_time = datetime.now(timezone.utc).timestamp()
+            document_generator = self.connector.poll_source(poll_start.timestamp(), end_time)
+            _begin_info = f"from {poll_start}"
+
+        self.log_connection(
+            "Slack",
+            f"channels({', '.join(channels) if channels else 'all'})",
+            task,
+        )
+        return document_generator
 
 
 class Teams(SyncBase):