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/locales/en.ts

@@ -1238,6 +1238,12 @@ Example: Virtual Hosted Style`,
         'Upload the OAuth JSON generated from Google Console. If it only contains client credentials, run the browser-based verification once to mint long-lived refresh tokens.',
       dropboxDescription:
         'Connect your Dropbox to sync files and folders from a chosen account.',
+      slackDescription:
+        'Connect your Slack workspace to sync channel messages and threads.',
+      slackBotTokenTip:
+        'Slack bot user OAuth token (starts with xoxb-). The app needs the channels:read, channels:history, and users:read scopes.',
+      slackChannelsTip:
+        'Optional: channel names to sync (e.g., general). Leave empty to sync all accessible channels.',
       sharepointDescription:
         'Connect a SharePoint site via Microsoft Graph to sync its document libraries.',
       sharepointSiteUrlTip:

web/src/locales/zh.ts

@@ -1099,6 +1099,11 @@ NER：使用 spaCy NER 和基于规则的关键词提取来抽取实体和关系
       gmailTokenTip:
         '请上传由 Google Console 生成的 OAuth JSON。如果仅包含 client credentials，请通过浏览器授权一次以获取长期有效的刷新 Token。',
       dropboxDescription: '连接 Dropbox，同步指定账号下的文件与文件夹。',
+      slackDescription: '连接你的 Slack 工作区，同步频道消息与讨论串。',
+      slackBotTokenTip:
+        'Slack 机器人用户 OAuth Token（以 xoxb- 开头）。应用需具备 channels:read、channels:history 和 users:read 权限。',
+      slackChannelsTip:
+        '可选：需要同步的频道名称（例如 general）。留空则同步所有可访问的频道。',
       sharepointDescription: '通过 Microsoft Graph 连接 SharePoint 站点，同步其文档库。',
       sharepointSiteUrlTip:
         '要索引的 SharePoint 站点完整 URL，例如 https://contoso.sharepoint.com/sites/MySite。需要具备 Sites.Read.All 与 Files.Read.All 应用权限（管理员同意）的 Azure AD 应用。',