GPUI's `bench-support` feature provides the benchmark context and
headless-renderer interface without enabling `test-support`, but the
platform implementation remained available only through
`gpui_platform/test-support`. As a result, consumers could not use
`#[gpui::bench]` with a fully isolated feature graph because the
generated benchmark harness calls
`gpui_platform::current_headless_renderer()`.

This threads `bench-support` through `gpui_platform`, `gpui_macos`, and
`gpui_apple`, exposing the existing Metal headless renderer without
widening unrelated test window or prompt APIs. The existing benchmark
package now requests the platform's canonical `bench-support` feature.
Its other test-backed fixtures are intentionally unchanged and remain
outside this focused dependency fix.

Validation:

- Confirmed `gpui_apple/bench-support`, `gpui_macos/bench-support`, and
`gpui_platform/bench-support` resolve no `test-support` feature with
`cargo tree --locked --no-default-features --features bench-support -e
no-dev,features`.
- Ran `cargo check --locked -p benchmarks --benches` to compile every
existing `#[gpui::bench]` target against the real platform headless
renderer.
- Ran `cargo nextest run --locked -p gpui_apple -p gpui_macos -p
gpui_platform`.
- Ran `./script/clippy` for each changed platform package with
`--no-default-features --features bench-support`.
- Ran `cargo fmt --check`.

Release Notes:

- N/A