fix(aigateway): add backend upstream test endpoint to avoid frontend CORS

[Rader]

Summary:

• Adds a new POST /config/upstreams/:id/test backend endpoint to resolve frontend CORS errors when testing commercial API upstreams.
• The backend securely fetches upstream configurations, sends a test request with a 30s timeout, and returns a sanitized summary including status codes and extracted content.
• Resolves issue #2310 by enabling reliable connection testing for both /chat/completions and /responses endpoint formats without exposing sensitive credentials.

[Rader]

Depends on #1243

[HaiHui886]

lgtm