When a model produces poor output during commit message generation,
there was no way to cancel it. This replaces the non-interactive spinner
with a Stop button that cancels the generation task. Partial generated
text is kept in the editor.

Partially addresses #33556


<img width="1200" height="900" alt="2026-02-18-160103_hyprshot"
src="https://github.com/user-attachments/assets/760e9681-b374-4f40-a4b1-0bb6775db17c"
/>

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Before you mark this PR as ready for review, make sure that you have:
- [x] Added a solid test coverage and/or screenshots from doing manual
testing
- [x] Done a self-review taking into account security and performance
aspects
- [x] Aligned any UI changes with the [UI
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)

Release Notes:

- Added a button to stop commit message generation in the git panel

---------

Co-authored-by: Danilo Leal <daniloleal09@gmail.com>
Co-authored-by: Danilo Leal <67129314+danilo-leal@users.noreply.github.com>