fix(chat): Improve Markdown formatter state management
- Explicitly clear the streaming formatter when switching conversations to prevent state leakage.
- Implement defensive checks to ensure the formatter is always associated with the correct message, resetting it if a mismatch is detected.
- Refine the formatter seeding logic to only use existing content in resume scenarios, preventing content duplication.
- Add detailed logging to the chat provider for better debugging of streaming and formatter behavior.
