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/routing.py

@@ -215,25 +215,27 @@ class ModelRouter:
                 continue
 
             if model_name not in available_models:
-                errors.append({
-                    "model": model_name,
-                    "error": f"Provider {config.provider.value} not configured",
-                })
+                errors.append(
+                    {
+                        "model": model_name,
+                        "error": f"Provider {config.provider.value} not configured",
+                    }
+                )
                 continue
 
             # Check circuit breaker
             circuit = self._circuit_registry.get_circuit_sync(config.provider.value)
             if not circuit.is_available():
-                errors.append({
-                    "model": model_name,
-                    "error": f"Circuit open for {config.provider.value}",
-                })
+                errors.append(
+                    {
+                        "model": model_name,
+                        "error": f"Circuit open for {config.provider.value}",
+                    }
+                )
                 continue
 
             # Model is available
-            logger.debug(
-                f"Selected model {model_name} for group {model_group.value}"
-            )
+            logger.debug(f"Selected model {model_name} for group {model_group.value}")
             return model_name, config
 
         # No models available