fix: preserve reasoning and thinking metadata in Message.fit

[l50]

Key Changes:

• Updated Message.fit to retain thinking_blocks and reasoning_content
• Added tests to ensure preservation of these metadata fields
• Improved robustness for handling objects with optional metadata fields

Added:

• Unit tests for Message.fit verifying that thinking_blocks and
  reasoning_content are correctly preserved or ignored as appropriate in
  various scenarios - tests/test_chat.py

Changed:

• Enhanced Message.fit method to detect and copy thinking_blocks and
  reasoning_content from input objects into the resulting Message's
  metadata only if they are present and non-empty - rigging/message.py
• Updated logic to ensure that empty or missing thinking_blocks and
  reasoning_content are not included in metadata, improving message object
  cleanliness and consistency

Fixes ENG-3882

Generated Summary

• Updated the Message.fit method to extract "thinking_blocks" and "reasoning_content" from input messages and include them in the metadata if they are provided.
• Merged additional metadata into the copied message instance when non-empty values are present.
• Added new tests to verify that:
  • "thinking_blocks" is preserved when provided.
  • "reasoning_content" is preserved when provided.
  • Both fields are correctly handled simultaneously.
  • Empty values for these fields are not added to the metadata.

This summary was generated with ❤️ by rigging