Let TestAppContext render with a real text system, renderer and scale

[francois-unity]

Adds TestAppContext::with_platform, which takes a text system, an asset source, a headless renderer factory and a scale factor, so a TestAppContext window renders to pixels through Window::render_to_image the way the app draws it. build, and with it #[gpui::test], keeps the stub text system, no renderer, no assets and a scale of 2.

Why: the native Unity Hub’s visual regression suite renders seeded scenes from its headless harness, which is built on TestAppContext and VisualTestContext (HUB-8114, for HUB-8065). build hardcodes TestPlatform::new: the stub TestTextSystem paints no glyphs, there is no renderer, and the empty AssetRegistry drops svg() icons. TestWindow::scale_factor is the constant 2. HeadlessAppContext has that configuration but isn’t a TestAppContext; it stays as it is.

• TestPlatform keeps a scale factor (2 by default) and hands it to each TestWindow, whose scale_factor() reads it, resizes included.
• gpui_platform::current_headless_renderer() returns WGPU’s headless renderer on Linux, FreeBSD and Windows under test-support, through an optional gpui_wgpu dependency that Linux already builds for its windows. Like Metal’s without a device, it panics without a GPU adapter; lavapipe or WARP is enough. Under bench-support alone it still returns None there.
• CI runs the new rendering test on all three platforms.

How to test

• cargo +1.98.1 test -p gpui-ce --lib -- app::test_context: three new tests. With a recording renderer, a with_platform window at scale 1.5 reports that scale, renders and reads back 300×150 for 200×100, keeps the scale through a resize, and loads its assets. build still gives scale 2, no glyph sprites and no assets, and render_to_image refuses. A zero scale panics.
• GPUI_RUN_RENDERING_TESTS=1 cargo +1.98.1 test -p gpui_ce_platform --features test-support --test test_app_context_render: IBM Plex through Parley with current_headless_renderer() at scales 1 and 1.5, checking glyph sprites, the image size, the background colour and the ink. Green with Metal on macOS; CI runs it with WGPU on Linux and Windows.
• cargo +1.98.1 test -p gpui-ce --features test-support, -p gpui_ce_elements -p gpui_ce_parley -p gpui_ce_macos -p gpui_ce_platform, parley_render and workspace clippy with -D warnings are green on macOS. A Linux-target cargo clippy -p gpui-ce -p gpui_ce_platform --all-targets -- -D warnings, with and without test-support, is clean.
• --features bench-support alone stops on two errors already at the base (bench_context.rs:536, debug_overlay.rs:220), so its None branch is checked by reading.
• As on Keep a paragraph’s closing whitespace on its last row and in its width #7, clippy, msrv, test-linux and test-windows stop on the base’s bindgen build-script error (Declare gpui_media’s bindgen build dependency on every host #6 fixes it), and typos on the base’s wdth (Let typos accept the renamed Noto Sans font file #10 fixes it).
• Unity Hub pins this commit on top of HUB-8070’s pin, unity/HUB-8070-hub-pin-2 (Read a Linux window’s frame back with render-to-image #9’s two commits over unity/HUB-8048-hub-pin-3), as unity/HUB-8114-hub-pin-2 (d12157734f) in Unity-Technologies/unity-hub#16239. The cherry-pick is clean, and the same tests and clippy runs are green there, with render-to-image on as well.