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/testcases/restful_api/test_documents.py

@@ -64,16 +64,19 @@ def test_documents_upload_and_list(rest_client, create_dataset, tmp_path):
     assert any(doc["name"] == fp.name for doc in list_payload["data"]["docs"]), list_payload
 
 
-def _upload_files(rest_client, dataset_id, file_paths):
+def _upload_files(rest_client, dataset_id, file_paths, timeout=None):
     with ExitStack() as stack:
         files = [("file", (fp.name, stack.enter_context(fp.open("rb")))) for fp in file_paths]
-        return rest_client.post(f"/datasets/{dataset_id}/documents", files=files)
+        kwargs = {"files": files}
+        if timeout is not None:
+            kwargs["timeout"] = timeout
+        return rest_client.post(f"/datasets/{dataset_id}/documents", **kwargs)
 
 
-def _seed_documents(rest_client, create_dataset, tmp_path, count=5):
+def _seed_documents(rest_client, create_dataset, tmp_path, count=5, timeout=None):
     dataset_id = create_dataset("dataset_list_contract")
     file_paths = [create_txt_file(tmp_path / f"ragflow_test_upload_{i}.txt") for i in range(count)]
-    res = _upload_files(rest_client, dataset_id, file_paths)
+    res = _upload_files(rest_client, dataset_id, file_paths, timeout=timeout)
     assert res.status_code == 200
     payload = res.json()
     assert payload["code"] == 0, payload
@@ -1166,7 +1169,9 @@ def test_documents_delete_invalid_dataset_partial_duplicate_repeat_and_cross_dat
 
 @pytest.mark.p2
 def test_documents_delete_concurrent_and_bulk_contract(rest_client, create_dataset, tmp_path):
-    dataset_id, uploaded_docs = _seed_documents(rest_client, create_dataset, tmp_path, count=60)
+    dataset_id, uploaded_docs = _seed_documents(
+        rest_client, create_dataset, tmp_path, count=60, timeout=120
+    )
     document_ids = [doc["id"] for doc in uploaded_docs]
 
     with ThreadPoolExecutor(max_workers=8) as executor:
@@ -1192,9 +1197,15 @@ def test_documents_delete_concurrent_and_bulk_contract(rest_client, create_datas
     assert list_after_payload["code"] == 0, list_after_payload
     assert list_after_payload["data"]["total"] == 0, list_after_payload
 
-    bulk_dataset_id, bulk_docs = _seed_documents(rest_client, create_dataset, tmp_path, count=120)
+    bulk_dataset_id, bulk_docs = _seed_documents(
+        rest_client, create_dataset, tmp_path, count=120, timeout=120
+    )
     bulk_ids = [doc["id"] for doc in bulk_docs]
-    bulk_delete_res = rest_client.delete(f"/datasets/{bulk_dataset_id}/documents", json={"ids": bulk_ids})
+    bulk_delete_res = rest_client.delete(
+        f"/datasets/{bulk_dataset_id}/documents",
+        json={"ids": bulk_ids},
+        timeout=120,
+    )
     assert bulk_delete_res.status_code == 200
     bulk_delete_payload = bulk_delete_res.json()
     assert bulk_delete_payload["code"] == 0, bulk_delete_payload