fix(llm-gateway): improve type safety and datetime consistency

- Add type annotations for mypy compliance
- Use UTC-aware datetimes consistently (datetime.now(UTC))
- Add type: ignore comments for LiteLLM incomplete stubs
- Fix import ordering and formatting
- Update pyproject.toml mypy configuration

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

mcp-servers/llm-gateway/tests/test_server.py

@@ -36,6 +36,7 @@ def test_client(test_settings: Settings) -> TestClient:
         mock_provider.return_value.get_available_models.return_value = {}
 
         from server import app
+
         return TestClient(app)
 
 
@@ -243,7 +244,9 @@ class TestListModelsTool:
         """Test listing models by group."""
         with patch("server.get_model_router") as mock_router:
             mock_router.return_value = MagicMock()
-            mock_router.return_value.parse_model_group.return_value = ModelGroup.REASONING
+            mock_router.return_value.parse_model_group.return_value = (
+                ModelGroup.REASONING
+            )
             mock_router.return_value.get_available_models_for_group = AsyncMock(
                 return_value=[]
             )
@@ -340,9 +343,7 @@ class TestChatCompletionTool:
                             "arguments": {
                                 "project_id": "proj-123",
                                 "agent_id": "agent-456",
-                                "messages": [
-                                    {"role": "user", "content": "Hello"}
-                                ],
+                                "messages": [{"role": "user", "content": "Hello"}],
                                 "stream": True,
                             },
                         },
@@ -388,7 +389,9 @@ class TestChatCompletionTool:
                         mock_circuit = MagicMock()
                         mock_circuit.record_success = AsyncMock()
                         mock_reg.return_value = MagicMock()
-                        mock_reg.return_value.get_circuit_sync.return_value = mock_circuit
+                        mock_reg.return_value.get_circuit_sync.return_value = (
+                            mock_circuit
+                        )
 
                         response = test_client.post(
                             "/mcp",