Fix type annotations and add missing logging in mplex and Quic

[Jineshbansal]

Fixes #1135

What was wrong?

Several TODOs in the codebase related to type safety and logging:

1. QUICErrorContext.__exit__ had incorrect type hints - exc_type had a duplicate | None and exc_tb was typed as Any instead of TracebackType | None

2. Mplex.send_message had return type -> int but the underlying write_to_stream() returns None, requiring a # type: ignore to suppress the error

3. Missing logging for edge cases in mplex message handling:

   • Unknown message flag
   • Message for non-existent stream
   • Data received after stream closed

How was it fixed?

• Added TracebackType import and corrected the __exit__ signature
• Changed send_message return type to -> None and removed the unnecessary return statement (verified no callers use the return value)
• Added logger.warning() calls for all three edge cases with descriptive messages

Testing

• mypy passes on modified files
• ruff linting passes
• All mplex tests pass (32/32)