PR #51415 introduced the `PlatformHeadlessRenderer` trait and
`PlatformWindow::render_to_image` method in `gpui`, both gated on
`#[cfg(any(test, feature = "test-support"))]`, plus corresponding impls
in `gpui_macos` (`window.rs` and `metal_renderer.rs`) gated on the same
cfg.

A dependent crate's `cfg(test)` flag does **not** propagate to its
dependencies. So when `cargo test -p gpui_macos` is run in isolation,
`gpui_macos`'s own `cfg(test)` is true (its impls get compiled) but
`gpui` is just a regular dependency without `test-support` enabled (the
trait and method don't exist), and the build fails:

```
error[E0405]: cannot find trait `PlatformHeadlessRenderer` in crate `gpui`
error[E0407]: method `render_to_image` is not a member of trait `PlatformWindow`
```

The fix is to enable `gpui/test-support` as a dev-dependency, so the
feature is on exactly when `gpui_macos`'s tests are being built.

This bug is latent on `main` — workspace-level `cargo test` typically
pulls in `gpui/test-support` transitively from other crates, masking it.
Running `cargo test -p gpui_macos` alone is what surfaces it.

Release Notes:

- N/A