Close [AI-395](https://linear.app/affine-design/issue/AI-395)
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## Summary by CodeRabbit
* **Bug Fixes**
* Improved chat stability by resetting chat action signals after
processing to prevent repeated triggers.
* **New Features**
* Added end-to-end tests for new chat session creation and chat pinning
functionality to enhance reliability.
* **Enhancements**
* Enhanced chat toolbar with test identifiers and pinned state
attributes for better accessibility and testing.
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Co-authored-by: fengmk2 <fengmk2@gmail.com>
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