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)

web/src/pages/user-setting/data-source/constant/index.tsx

@@ -43,9 +43,9 @@ export enum DataSourceKey {
   POSTGRESQL = 'postgresql',
   REST_API = 'rest_api',
   RSS = 'rss',
+  SLACK = 'slack',
   SHAREPOINT = 'sharepoint',
 
-  //   SLACK = 'slack',
   //   TEAMS = 'teams',
 }
 
@@ -130,6 +130,12 @@ export const DataSourceFeatureVisibilityMap: Partial<
   [DataSourceKey.MOODLE]: {
     syncDeletedFiles: true,
   },
+  [DataSourceKey.SLACK]: {
+    syncDeletedFiles: true,
+  },
+  [DataSourceKey.SHAREPOINT]: {
+    syncDeletedFiles: true,
+  },
   [DataSourceKey.MYSQL]: {
     syncDeletedFiles: true,
   },
@@ -213,6 +219,11 @@ export const generateDataSourceInfo = (t: TFunction) => {
       description: t(`setting.${DataSourceKey.MOODLE}Description`),
       icon: <SvgIcon name={'data-source/moodle'} width={38} />,
     },
+    [DataSourceKey.SLACK]: {
+      name: 'Slack',
+      description: t(`setting.${DataSourceKey.SLACK}Description`),
+      icon: <SvgIcon name={'data-source/slack'} width={38} />,
+    },
     [DataSourceKey.SHAREPOINT]: {
       name: 'SharePoint',
       description: t(`setting.${DataSourceKey.SHAREPOINT}Description`),
@@ -659,6 +670,22 @@ export const DataSourceFormFields = {
       required: true,
     },
   ],
+  [DataSourceKey.SLACK]: [
+    {
+      label: 'Slack Bot Token',
+      name: 'config.credentials.slack_bot_token',
+      type: FormFieldType.Password,
+      required: true,
+      tooltip: t('setting.slackBotTokenTip'),
+    },
+    {
+      label: 'Channels',
+      name: 'config.channels',
+      type: FormFieldType.Tag,
+      required: false,
+      tooltip: t('setting.slackChannelsTip'),
+    },
+  ],
   [DataSourceKey.SHAREPOINT]: [
     {
       label: 'Site URL',
@@ -1542,6 +1569,16 @@ export const DataSourceFormDefaultValues = {
       },
     },
   },
+  [DataSourceKey.SLACK]: {
+    name: '',
+    source: DataSourceKey.SLACK,
+    config: {
+      channels: [],
+      credentials: {
+        slack_bot_token: '',
+      },
+    },
+  },
   [DataSourceKey.SHAREPOINT]: {
     name: '',
     source: DataSourceKey.SHAREPOINT,