improve: add exc_info to MoA error logging

[aydnOktay]

This PR improves error observability in the Mixture-of-Agents (MoA) tool by adding exc_info=True to all error and warning log calls, ensuring full stack traces are captured for debugging.

Changes :

Added exc_info=True to all logger.warning calls in _run_reference_model_safe for invalid requests, rate limits, and unknown errors
Added exc_info=True to the final logger.error call when all retries are exhausted
Added exception handling to _run_aggregator_model with proper error logging using exc_info=True
Added exc_info=True to the main error handler in mixture_of_agents_tool

This ensures that when MoA processing fails, developers get complete stack traces in logs, making it easier to diagnose API issues, network problems, or unexpected errors during multi-model processing.