feat(go-api): implement connector (data source) management endpoints (#15274)

## 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.

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Co-authored-by: Jin Hai <haijin.chn@gmail.com>