agent_ui: Add button to delete all threads

[linuxsoares]

This pull request introduces a new "Remove All History" feature to the agent UI, allowing users to delete all history threads at once. The most important changes are grouped below:

Feature Implementation:

• Added a new RemoveHistory action and handler to AcpThreadView, enabling the deletion of all history threads via history_store.delete_threads(cx) [1] [2].
• Updated the UI to include a "Remove All" button in the recent history section, which dispatches the RemoveHistory action when clicked [1] [2].

Action Handling and Integration:

• Registered the remove_all_history action handler in the main action dispatch for AcpThreadView to ensure the action is processed correctly.
• Modified the construction of certain components to use project.clone() instead of project.downgrade() for consistency [1] [2].

Testing:

• Added extensive tests for the "Remove All History" feature in both thread_view.rs and thread_history.rs, covering scenarios such as empty history, existing threads, button click, session preservation, and confirmation state resets [1] [2].
• Included necessary test imports and setup helpers to facilitate the new test cases.

These changes collectively provide a robust and user-friendly way to clear all history threads from the agent UI, with thorough validation to ensure reliability.

[cla-bot]

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[zed-industries-bot]

	Warnings
⚠️	This PR is missing release notes. Please add a "Release Notes" section that describes the change: Release Notes: - Added/Fixed/Improved ... If your change is not user-facing, you can use "N/A" for the entry: Release Notes: - N/A

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[danilo-leal]

Hey, thanks for working on this! Curious how is this different from #43370?

[danilo-leal]

I'll close this PR in the meantime; we already support this feature as per the linked PR above. Ping me if I'm missing anything here. Thank you!