fix(security): fail closed in /api/models auth gate on unexpected errors (#3489)

GET /api/models swallowed any non-HTTPException raised while checking
whether the caller is authenticated (bare except Exception: pass), so a
broken auth_manager or an exception from get_current_user silently
granted the full model list to an anonymous caller instead of rejecting
the request. Now any unexpected exception logs and returns HTTP 500.

Split out of #2360 per reviewer request to keep the deny-list and the
auth-gate fix as separate, single-purpose PRs.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

routes/model_routes.py

@@ -1232,8 +1232,9 @@ def setup_model_routes(model_discovery):
                 raise HTTPException(401, "Not authenticated")
         except HTTPException:
             raise
-        except Exception:
-            pass
+        except Exception as e:
+            logger.error('Auth gate error in GET /api/models, failing closed: %s', e)
+            raise HTTPException(status_code=500, detail='Internal error')
         # Admins see every endpoint (they manage the global pool); regular
         # users get the owner-scoped view.
         _is_admin = False