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/cost_tracking.py

@@ -233,7 +233,7 @@ class CostTracker:
     ) -> UsageReport:
         """Get usage report from a Redis hash."""
         r = await self._get_redis()
-        data = await r.hgetall(key)
+        data = await r.hgetall(key)  # type: ignore[misc]
 
         # Parse the hash data
         by_model: dict[str, dict[str, Any]] = {}
@@ -311,7 +311,7 @@ class CostTracker:
         """
         r = await self._get_redis()
         key = f"{self._prefix}:cost:session:{session_id}"
-        data = await r.hgetall(key)
+        data = await r.hgetall(key)  # type: ignore[misc]
 
         # Parse similar to _get_usage_report
         result: dict[str, Any] = {