fix: resolve remaining Copilot review comments

- Return browser-facing origin URL in /config API_URL instead of empty
  string, so health checks and other API calls route correctly through
  the frontend reverse proxy
- Use error-specific WebSocket close codes (1011 for backend errors,
  1000 for normal closure) instead of a blanket close
- Remove /config proxy from vite dev server to avoid failures when
  frontend_server is not running (frontend falls back to defaults)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Ashwal-Microsoft

src/frontend/frontend_server.py

@@ -58,13 +58,13 @@ async def serve_index():
 
 
 @app.get("/config")
-async def get_config():
+async def get_config(request: Request):
+    # The browser receives the frontend's own origin as the API base so that
+    # all /api/* requests (including /api/health) route through the frontend
+    # reverse proxy rather than hitting the internal backend directly.
+    browser_api_url = str(request.base_url).rstrip("/")
     config = {
-        # Return empty string so the browser uses relative /api/* paths
-        # which are proxied server-side to BACKEND_API_URL. This ensures
-        # backend Container Apps with internal-only ingress are never
-        # contacted directly from the browser.
-        "API_URL": "",
+        "API_URL": browser_api_url,
         "REACT_APP_MSAL_AUTH_CLIENTID": os.getenv(
             "REACT_APP_MSAL_AUTH_CLIENTID", "Client ID not set"
         ),
@@ -133,7 +133,11 @@ async def forward_to_client():
             await asyncio.gather(*pending, return_exceptions=True)
     except Exception as exc:
         logger.error("WebSocket proxy error for batch %s: %s", batch_id, exc)
-    finally:
+        try:
+            await websocket.close(code=1011, reason="backend connection error")
+        except Exception:
+            pass
+    else:
         try:
             await websocket.close(code=1000, reason="proxy closed")
         except Exception:

src/frontend/vite.config.js

@@ -10,14 +10,6 @@ export default defineConfig({
       '/api': {
         target: 'http://localhost:8000',
         changeOrigin: true,
-      },
-      '/config': {
-        target: 'http://localhost:3000',
-        changeOrigin: true,
-        // Only proxied when the Python frontend_server is running locally
-        configure: (proxy) => {
-          proxy.on('error', () => {});
-        }
       }
     }
   }