phsl / new-api

BREAKING CHANGE: Refactor message stream processing and think tag handling logic

- Improve automatic collapse behavior of reasoning panel
  - Collapse panel immediately after reasoning content completion
  - Add detection for complete think tag pairs
  - Remove dependency on full content completion

- Enhance think tag processing
  - Unify tag handling logic across stream and stop states
  - Add robust handling for unclosed think tags
  - Prevent markdown rendering errors from malformed tags

- Simplify state management
  - Reduce complexity in collapse condition checks
  - Remove redundant state transitions
  - Improve code readability by removing unnecessary comments

This commit ensures a more stable and predictable behavior in the chat interface, particularly in handling streaming responses and reasoning content display.