Fix flaky CI: update deprecated model, filter leaked async task logs

- test_router_context_window_check_pre_call_check_out_group: replace deprecated
  gpt-3.5-turbo-1106 (removed from model_cost, returns max_input_tokens=0) with
  gpt-4.1-mini + mock_response
- test_async_fallbacks: filter "Task was destroyed but it is pending" messages
  that leak from parallel test execution in CI

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

tests/local_testing/test_router.py

@@ -825,8 +825,9 @@ def test_router_context_window_check_pre_call_check_out_group():
             {
                 "model_name": "gpt-3.5-turbo-large",  # openai model name
                 "litellm_params": {  # params for litellm completion/embedding call
-                    "model": "gpt-3.5-turbo-1106",
+                    "model": "gpt-4.1-mini",
                     "api_key": os.getenv("OPENAI_API_KEY"),
+                    "mock_response": "Alexander was a great conqueror.",
                 },
             },
         ]

tests/local_testing/test_router_debug_logs.py

@@ -83,6 +83,7 @@ def test_async_fallbacks(caplog):
         log
         for log in captured_logs
         if "Task exception was never retrieved" not in log
+        and "Task was destroyed but it is pending" not in log
         and "get_available_deployment" not in log
         and "in the Langfuse queue" not in log
     ]