fix(tests): add cache flush fixture for reliable HTTP client isolation

Add _reset_litellm_http_client_cache autouse fixture (matching
test_vertex_gemma_transformation.py) to flush in_memory_llm_clients_cache
before each test. Without this, a cached real AsyncHTTPHandler from an
earlier test could bypass the class-level mock and cause real HTTP calls.

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

tests/test_litellm/llms/vertex_ai/vertex_ai_partner_models/gpt_oss/test_vertex_ai_gpt_oss_transformation.py

@@ -15,6 +15,14 @@ from litellm.llms.vertex_ai.vertex_ai_partner_models.gpt_oss.transformation impo
 )
 
 
+@pytest.fixture(autouse=True)
+def _reset_litellm_http_client_cache():
+    """Ensure each test gets a fresh async HTTP client mock."""
+    from litellm import in_memory_llm_clients_cache
+
+    in_memory_llm_clients_cache.flush_cache()
+
+
 @pytest.fixture(autouse=True)
 def clean_vertex_env():
     """Clear Google/Vertex AI environment variables before each test to prevent test isolation issues."""