native_menu: Prevent CWD files from shadowing relative icons (#2814)

Closes #2813

## Description

Built-in native-menu icons use relative asset paths such as
`icons/close.svg`. On macOS and Windows, `resolve_icon_image` previously
checked whether that relative path existed on the filesystem before
consulting the application `AssetSource`. This allowed a matching file
under the process current working directory to shadow a bundled icon.

This PR limits filesystem-backed native-menu icons to explicit absolute
paths. Relative paths are now always resolved through `AssetSource`,
while existing support for absolute custom icon files is preserved.

The change is intentionally scoped to the native-menu resolver and adds
two target-gated regression tests covering three resolution properties:

- Relative paths resolve only through `AssetSource`, including when a
matching CWD file exists or the asset is missing.
- An explicit absolute icon path still resolves from the filesystem.

## Screenshot

| Before | After |
| ------ | ----- |
| ![Before fix: matching CWD file
wins](https://github.com/lurenjia534/gpui-component/releases/download/native-menu-icon-cwd-shadow-evidence-20260824/native-menu-icon-cwd-shadow-poc.png)
| ![After fix: relative asset wins and no CWD fallback
occurs](https://github.com/lurenjia534/gpui-component/releases/download/native-menu-icon-cwd-shadow-evidence-20260824/native-menu-icon-cwd-shadow-fix-verified.png)
|

The before screenshot shows the attacker-controlled CWD file being
selected. The after screenshot shows that the same shadow file exists
but the embedded asset is selected instead.

The screenshots were captured from a Linux logic-level PoC because the
native-menu image resolver is compiled only on macOS and Windows. The
committed regression tests exercise the real resolver on those affected
platforms in CI.

## How to Test

On macOS or Windows, run the native-menu tests:

```bash
cargo test -p gpui-component native_menu::tests::
```

The following relevant tests should pass:

```text
test_relative_icon_path_only_uses_asset_source
test_absolute_icon_path_loads_from_filesystem
```

Local Linux verification completed:

```bash
cargo clippy -p gpui-component --lib -- --deny warnings
cargo test -p gpui-component --lib
```

A separate post-fix logic PoC also confirmed that a matching CWD file
was present while relative assets did not fall back to it and absolute
file paths continued to work.

macOS and Windows native-menu execution was not available in the local
Linux environment; the repository CI matrix runs the test suite on both
affected platforms.

## AI Assistance

AI assistance was used to analyze the reported issue, minimize the
supplied patch, adjust the documentation and regression tests, run local
verification, and draft this PR description. The resulting code and
claims were reviewed before submission.

## Checklist

- [x] I have read the [CONTRIBUTING](../CONTRIBUTING.md) document and
followed the guidelines.
- [x] Reviewed the changes in this PR and confirmed AI generated code
(if any) is accurate.
- [ ] Passed `cargo run` for story tests related to the changes.
- [ ] Tested macOS, Windows and Linux platforms performance (if the
change is platform-specific).

The Story test is not applicable to this resolver-only change. The
changed module passes `rustfmt`; Linux Clippy and library tests passed
locally. The new resolver tests are target-gated to macOS and Windows
and are covered by CI.

---------

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