test(platform): catch a platform-only dependency used from other-platform code

[kirillDevPro]

What & why

raw-window-handle was declared only for Windows while macOS-only code (panels/chart/shot/mac.rs) used it. Windows CI stayed green and the macOS release build failed with E0433 (fixed in #978).

This adds a source-text contract to theme_contract that runs in a plain cargo test on any host: it walks the real module tree and its #[cfg(...)] gates and fails when gated code references an external crate that Cargo.toml does not make available for that target. The message names the file, the crate and the table to add it to.

How to verify

• cargo test -p moon-ui-gpui --test theme_contract passes on the current tree.
• Deleting the macOS raw-window-handle line makes platform_dependencies_cover_reachable_source fail naming shot/mac.rs and raw_window_handle; restoring it turns it green.