tab: Add max_width to TabBar (#2578)

## Description

Adds `max_tab_width` to `TabBar`, capping each tab rendered width.
Labels set via `.label()` truncate with an ellipsis past the limit;
icon-only tabs are unaffected. Tabs with `.child()` content get the
width constraint but handle their own truncation. The overflow menu
still shows full labels.

## Screenshot

| Before | After |
| ------ | ----- |
| <img width="1712" height="1242" alt="image"
src="https://github.com/user-attachments/assets/d7a1d81e-7aed-4889-835b-9c7b980a571f"
/> | <img width="1712" height="1242" alt="image"
src="https://github.com/user-attachments/assets/82630c9d-54e6-4253-bb90-4f4cd489eb42"
/> |

## How to Test

1. `cargo run` and open the Tabs story.
2. Toggle “Max tab width”; the enabled value is fixed at 110px.
3. Confirm long labels truncate, short labels keep their intrinsic
width, custom children remain constrained, and the overflow menu shows
full text.
4. Run `cargo test -p gpui-component tab::`.

## Checklist

- [x] Read CONTRIBUTING and followed the guidelines.
- [x] Reviewed changes, confirmed AI-generated code is accurate.
- [x] Passed Story compilation and tab tests.
- [ ] Tested macOS, Windows, Linux performance (if platform-specific).

---------

Co-authored-by: Jason Lee <huacnlee@gmail.com>
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>