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

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

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

@@ -2,7 +2,6 @@
 Tests for exceptions module.
 """
 
-
 from exceptions import (
     AllProvidersFailedError,
     CircuitOpenError,