fix(css): retry scss compilation when the embedded sass compiler fails to spawn

[lopno]

Description

Fixes #23374. Related: #20825 (closed, locked).

When sass-embedded's compiler child process fails to spawn (e.g. intermittent spawn EBADF on macOS under fd pressure), the scss worker currently caches the dead compiler forever: initAsyncCompiler() resolves before the spawn settles, so the failure only surfaces per-compileStringAsync call, and compilerPromise ??= never re-runs. One lost race means every scss compile fails until the dev server is restarted.

This change retries once: if a compile rejects with a process-spawn error (err.syscall starting with spawn — real sass errors don't have it and are rethrown unchanged), the cached compiler is dropped and disposed fire-and-forget, and the compile runs again against a freshly spawned process.

Alternatives considered: retrying inside initAsyncCompiler isn't possible from Vite's side (it never rejects), and resetting only in stop() doesn't help a long-lived server. Scoping the retry to syscall: 'spawn*' errors keeps deliberate failures (bad scss, missing files) untouched.

Tests: not included — exercising this requires making sass-embedded's internal child_process.spawn fail once, and neither makeScssWorker nor a spawn seam is exported. I validated the equivalent retry logic against a stubbed compiler externally (single shared respawn under concurrent failures, sass errors pass through with zero retries), plus pnpm -F vite build, typecheck, and the css unit tests. Happy to add a test if you can suggest a seam you'd accept.

AI disclosure (per the AI policy): this fix was developed with AI assistance under my direction, including the root-cause investigation (module-resolution traces and reproduction of the EBADF cascade against a large app). I reviewed the final diff line by line; the retry scoping (syscall gate, single retry, fire-and-forget dispose of the dead compiler) reflects deliberate choices to keep the change minimal and never mask real compile errors.