Test chat agentic undo/redo

[connor4312]

• anyOS @sandy081
• anyOS @pierceboggan

Complexity: 4

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In this iteration we introduced the ability to undo or redo parts steps in responses, rather than entire responses. This is used in Agent mode to allow undoing single interactions.

1. In agent mode, make a request that causes the model to edit multiple files
2. Verify you see the diffs on code pills in the response
3. Verify clicking on the diffs will show the individual change made in that step, rather than the entire diff
4. Verify undo/redo works, and that you can reload the window and undo/redo parts of the response
5. Verify the model knows about undone steps. (Though this can be hard to get the model to realize as it often things it's stated intention was carried out even if some of the changes were rolled back)
6. It's only expected that response content and file edits are rolled back. Other tool calls like terminal commands are not rolled-back at this time.

Known issues:

• Diff stats will only appear once the response ends, pending https://github.com/microsoft/vscode-copilot/issues/13229)
• When a file is edited, all pills for that file will show as "editing" in the last response.