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_models.py

@@ -172,7 +172,10 @@ class TestChatMessage:
             role="user",
             content=[
                 {"type": "text", "text": "What's in this image?"},
-                {"type": "image_url", "image_url": {"url": "http://example.com/img.jpg"}},
+                {
+                    "type": "image_url",
+                    "image_url": {"url": "http://example.com/img.jpg"},
+                },
             ],
         )
         assert isinstance(msg.content, list)