git_panel: Add collapsable sections (#62441)

# Objective

- Make Git Panel grouping sections collapsible so users can hide
sections they are not currently interested in.
- Support collapsible sections when grouping changes by tracking and by
staging.
- Provide a clear visual indicator showing whether each section is
expanded or collapsed.

## Solution

- Added per-section collapsed state to the Git Panel.
- Made grouping headers clickable to toggle their section between
expanded and collapsed.
- Added chevron indicators that point down when expanded and right when
collapsed.
- Applied the behavior to both flat and tree views.
- Kept the stage/unstage checkbox independent from the header collapse
interaction.
- Ensured a previously collapsed Tracked section does not hide files
after switching to Group by None.

## Testing

- Ran `rustfmt --edition 2024 crates/git_ui/src/git_panel.rs --check`.
- Ran `cargo check -p git_ui`.
- Ran `cargo test -p git_ui`:
  - 129 tests passed
  - 0 tests failed
- Ran `git diff --check`.
- Attempted `./script/clippy -p git_ui` twice. It produced no lint
diagnostics but did not finish compiling the release, all-targets,
all-features dependency graph within the available timeout.
- Manually verify by selecting both grouping modes in the Git Panel and
clicking each section header in flat and tree views. Confirm that:
  - The section contents are hidden and restored.
  - The chevron updates to reflect the current state.
  - Clicking the stage/unstage checkbox does not collapse the section.
  - Switching to Group by None does not leave tracked files hidden.

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

## Showcase



https://github.com/user-attachments/assets/77ec4c88-4a6d-4e49-ac9f-ddbf80e724ca


---

Release Notes:

- Improved Git Panel organization by allowing grouped change sections to
be collapsed