### What problem does this PR solve?
The following API is available in go
> /api/v1/connectors/google/oauth/web/start POST
> /api/v1/connectors/gmail/oauth/web/callback GET
> /api/v1/connectors/google-drive/oauth/web/callback GET
> /api/v1/connectors/google/oauth/web/result POST
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
### What problem does this PR solve?
As title
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
- [x] New Feature (non-breaking change which adds functionality)
- [x] Refactoring
## Summary
Ports the connector (data source) management endpoints that power
`web/src/pages/user-setting/data-source/` from Python
(`api/apps/restful_apis/connector_api.py`) to Go. Previously only `GET
/connectors` (list) was implemented in Go; this adds the rest of the
lifecycle.
Closes#15273 (subtask of #15240).
## Endpoints implemented
All under base path `/api/v1` (mirrors the Python routes):
| Method | Path | Description |
|--------|------|-------------|
| POST | `/connectors/{connector_id}/test` | Validate stored credentials
|
`GET /connectors` (list) was already present and is unchanged.
---------
Co-authored-by: Jin Hai <haijin.chn@gmail.com>
### What problem does this PR solve?
implement delete, rebuild api for connector
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
### What problem does this PR solve?
implement create_connector API
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
## Summary
- Add Go REST support for `GET /api/v1/connectors/:connector_id`.
- Reuse the Python API behavior by returning the connector only when the
current user can access its tenant.
- Add focused handler coverage for success and unauthorized responses.
Co-authored-by: Jin Hai <haijin.chn@gmail.com>
