feat: implement Microsoft Teams data source connector (#15193)

### What problem does this PR solve?

Closes #15191.

RAGFlow shipped a Microsoft Teams connector stub
(`common/data_source/teams_connector.py`) whose document-loading methods
all returned `[]`, `Teams._generate()` was a `pass`, and Teams was
commented out of the data-source settings UI. As a result there was no
way to index Teams channel conversations into a knowledge base.

This PR implements the connector end to end on top of Microsoft Graph
(Office365-REST-Python-Client). It shares the MSAL client-credentials
auth shape with the SharePoint connector.

**Backend**

- `common/data_source/teams_connector.py`
- `load_credentials()` now builds the Graph client using an MSAL
client-credentials **token callback** — the form `GraphClient` actually
expects. (The previous stub passed a raw access-token string to
`GraphClient(...)`, which is not how that client is driven.) Token
acquisition is lazy, so credential loading performs no network call.
  - `validate_connector_settings()` lists teams via Graph.
- `load_from_checkpoint()` is now a generator that pages teams →
channels → messages, flattens each top-level post together with its
replies into one blob-based `Document` (`extension` `.txt`/`.html`,
`blob`, `size_bytes`, `doc_updated_at`). Incremental syncs are bounded
by message `lastModifiedDateTime` (falling back to `createdDateTime`).
Per-message errors surface as `ConnectorFailure` instead of aborting the
run.
- `retrieve_all_slim_docs_perm_sync()` yields id-only `SlimDocument`
batches and the checkpoint helpers return proper `TeamsCheckpoint`s.
- ACL → `ExternalAccess` mapping is intentionally left best-effort
(`load_from_checkpoint_with_perm_sync` delegates to the standard load)
because the sync pipeline does not currently persist `ExternalAccess`.
- `rag/svr/sync_data_source.py`
- Implemented `Teams._generate()` using the existing
`CheckpointOutputWrapper` pattern (same shape as Confluence/Jira/Google
Drive), supporting full reindex and incremental polling from
`poll_range_start`.
- `TeamsConnector` is already exported from
`common/data_source/__init__.py`.

**Frontend (`web/`)**

- Enabled the `TEAMS` data-source enum and added its form fields
(`tenant_id`, `client_id`, `client_secret`), default values, display
metadata, and a Teams icon.
- Added `teamsDescription` / `teamsTenantIdTip` to `en.ts` and `zh.ts`.

**Tests**

- `test/unit_test/data_source/test_teams_connector_unit.py`: mock-based
unit tests covering credential loading (incomplete creds raise, happy
path sets the Graph client, fetch-without-creds raises), post/reply
flattening (incl. the HTML vs text extension), incremental
`lastModifiedDateTime` filtering, and slim-doc listing. All 6 pass;
`ruff check` is clean.

### Type of change

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

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

@@ -43,10 +43,9 @@ export enum DataSourceKey {
   POSTGRESQL = 'postgresql',
   REST_API = 'rest_api',
   RSS = 'rss',
+  TEAMS = 'teams',
   SLACK = 'slack',
   SHAREPOINT = 'sharepoint',
-
-  //   TEAMS = 'teams',
 }
 
 type DataSourceFeatureVisibility = {
@@ -130,6 +129,9 @@ export const DataSourceFeatureVisibilityMap: Partial<
   [DataSourceKey.MOODLE]: {
     syncDeletedFiles: true,
   },
+  [DataSourceKey.TEAMS]: {
+    syncDeletedFiles: true,
+  },
   [DataSourceKey.SLACK]: {
     syncDeletedFiles: true,
   },
@@ -219,6 +221,11 @@ export const generateDataSourceInfo = (t: TFunction) => {
       description: t(`setting.${DataSourceKey.MOODLE}Description`),
       icon: <SvgIcon name={'data-source/moodle'} width={38} />,
     },
+    [DataSourceKey.TEAMS]: {
+      name: 'Microsoft Teams',
+      description: t(`setting.${DataSourceKey.TEAMS}Description`),
+      icon: <SvgIcon name={'data-source/teams'} width={38} />,
+    },
     [DataSourceKey.SLACK]: {
       name: 'Slack',
       description: t(`setting.${DataSourceKey.SLACK}Description`),
@@ -670,6 +677,27 @@ export const DataSourceFormFields = {
       required: true,
     },
   ],
+  [DataSourceKey.TEAMS]: [
+    {
+      label: 'Tenant ID',
+      name: 'config.credentials.tenant_id',
+      type: FormFieldType.Text,
+      required: true,
+      tooltip: t('setting.teamsTenantIdTip'),
+    },
+    {
+      label: 'Client ID',
+      name: 'config.credentials.client_id',
+      type: FormFieldType.Text,
+      required: true,
+    },
+    {
+      label: 'Client Secret',
+      name: 'config.credentials.client_secret',
+      type: FormFieldType.Password,
+      required: true,
+    },
+  ],
   [DataSourceKey.SLACK]: [
     {
       label: 'Slack Bot Token',
@@ -1569,6 +1597,17 @@ export const DataSourceFormDefaultValues = {
       },
     },
   },
+  [DataSourceKey.TEAMS]: {
+    name: '',
+    source: DataSourceKey.TEAMS,
+    config: {
+      credentials: {
+        tenant_id: '',
+        client_id: '',
+        client_secret: '',
+      }
+    }
+  },
   [DataSourceKey.SLACK]: {
     name: '',
     source: DataSourceKey.SLACK,