fix: use correct google-genai async API and remove debug endpoint

The google-genai SDK uses `client.aio.models.generate_content()` for
async calls, not `client.models.generate_content_async()` which doesn't
exist. Also removes the temporary /ai/provider-debug endpoint.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/core/ai_provider.py

@@ -68,7 +68,7 @@ class GeminiProvider(AIProvider):
             response_mime_type="application/json",
         )
 
-        response = await client.models.generate_content_async(
+        response = await client.aio.models.generate_content(
             model=self._model,
             contents=contents,
             config=config,