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/failover.py

@@ -8,7 +8,7 @@ temporarily disabling providers that are experiencing issues.
 import asyncio
 import logging
 import time
-from collections.abc import Callable
+from collections.abc import Awaitable, Callable
 from dataclasses import dataclass, field
 from enum import Enum
 from typing import Any, TypeVar
@@ -172,7 +172,7 @@ class CircuitBreaker:
 
     async def execute(
         self,
-        func: Callable[..., T],
+        func: Callable[..., Awaitable[T]],
         *args: Any,
         **kwargs: Any,
     ) -> T: