feat: auto-expand last reasoning block and collapse when no longer last

[roomote]

This PR implements the feature requested in #6309 to automatically expand the last reasoning block so users can see the model's thinking process as it happens.

Changes Made

• Auto-expand last reasoning block: The reasoning block is now initialized as expanded when it is the last message in the chat
• Auto-collapse when no longer last: Added a useEffect hook that automatically collapses reasoning blocks when they are no longer the last message
• Comprehensive tests: Added unit tests to verify the new behavior works correctly

Implementation Details

The implementation follows the approach suggested in the issue:

• Modified the initial state of reasoningCollapsed in ChatRow.tsx to check if the message is the last one and is a reasoning message
• Added a useEffect that monitors changes to the isLast prop and collapses reasoning blocks when they're no longer the last message

Testing

• All existing tests pass ✅
• New tests added specifically for this feature ✅
• Linting and type checking pass ✅

Acceptance Criteria Met

✅ Given the reasoning block is the last message, then the reasoning block default state is expanded
✅ Given the reasoning block is no longer the last message, then the reasoning block automatically collapses

Fixes #6309

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Important

This PR auto-expands the last reasoning block in a chat and collapses it when it's no longer the last, with comprehensive tests added.

• Behavior:
  • Auto-expand last reasoning block in ChatRow.tsx when it is the last message.
  • Auto-collapse reasoning block in ChatRow.tsx when it is no longer the last message.
• Implementation:
  • Modified reasoningCollapsed state initialization in ChatRow.tsx to check if the message is the last and is a reasoning message.
  • Added useEffect in ChatRow.tsx to monitor isLast prop changes and collapse reasoning blocks accordingly.
• Testing:
  • Added unit tests in ChatRow.spec.tsx to verify auto-expand and auto-collapse behavior.
  • Tests ensure non-reasoning messages are unaffected and manual toggle of reasoning block is possible.

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[diwakar-s-maurya]

I tested it. It works as expected.

[daniel-lxs]

Closing, please see #6309 (comment)

(Also this PR didn't work for me at all)