phsl / new-api

This commit improves the handling of message generation interruption in the
playground chat interface, ensuring both content and thinking process are
properly preserved.

Key changes:
- Preserve existing content when stopping message generation
- Handle both direct reasoningContent and <think> tag formats
- Extract and merge thinking process from unclosed <think> tags
- Maintain consistent thinking chain format with separators
- Auto-collapse reasoning panel after stopping for cleaner UI

Technical details:
- Modified onStopGenerator to properly handle SSE connection closure
- Added regex pattern to extract thinking content from <think> tags
- Implemented proper state management for incomplete messages
- Ensured all content types are preserved in their respective fields

This fix resolves the issue where thinking chain content would be lost
when stopping message generation mid-stream.