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/exceptions.py

@@ -75,7 +75,7 @@ class LLMGatewayError(Exception):
 
     def to_dict(self) -> dict[str, Any]:
         """Convert error to dictionary for JSON response."""
-        result = {
+        result: dict[str, Any] = {
             "error": self.code.value,
             "message": self.message,
         }
@@ -164,9 +164,7 @@ class RateLimitError(LLMGatewayError):
             error_details["retry_after_seconds"] = retry_after
 
         code = (
-            ErrorCode.PROVIDER_RATE_LIMIT
-            if provider
-            else ErrorCode.RATE_LIMIT_EXCEEDED
+            ErrorCode.PROVIDER_RATE_LIMIT if provider else ErrorCode.RATE_LIMIT_EXCEEDED
         )
 
         super().__init__(