[inureyes]

Closes #1747.

An audio-only server answers every audio route correctly and never reports itself ready, so /health fails any container probe pointed at it.

The defect

health_check decides readiness with two arms: side_model_ready(&state) when embedding_serving_mode.blocks_generation(), and model_provider.is_loaded() otherwise. A server started with a primary audio checkpoint takes the second arm, because an audio-only mode does not set a generation-blocking embedding mode. In that configuration there is no chat worker to load: startup.rs detects ModelType::Whisper or ModelType::Kokoro and populates the audio slot instead, while the deliberately unused chat worker records a terminal state. is_loaded() therefore stays false for the life of the process and /health returns the b10621 loading envelope forever.

That contradicts two documented promises at once. docs/server-features.md says GET /health and /v1/health provide readiness, and docs/audio-api.md documents -m <whisper checkpoint> and -m <kokoro checkpoint> as the way to serve audio. A documented configuration cannot pass its documented readiness probe, so an orchestrator or load balancer in front of it restarts or drains a server whose transcription, translation and speech routes are all serving.

The fix

A third arm: when the chat provider is unavailable and an audio provider is installed, report ready.

This cannot produce a false ready. startup.rs sets audio_model to Some(..) only after WhisperSttProvider::load(..) or KokoroTtsProvider::load(..) returns Ok, and .with_audio_model(..) runs before the HTTP listener binds. When the audio load fails the slot stays None, the new arm does not fire, and the server correctly reports not ready.

It also cannot mask a chat worker crash. audio_model is populated only when the primary checkpoint is itself an audio model, which is the deliberate case rather than the failure case. exited_chat_worker_returns_503_without_channel_details covers the crash case and is unchanged by this PR.

Verification

• audio_only_server_reports_healthy_after_its_worker_is_ready asserts 200 with exactly {"status": "ok"} for an audio-only state, alongside the existing audio_only_server_names_the_served_routes. The get helper it needs mirrors the post helper already in the file.
• Negative control: with the new arm reverted and the test kept, the test fails. It guards the fix rather than restating it.
• cargo test -p mlxcel --lib --features metal,accelerate availability passes 7 of 7 on an M1 Ultra, including the two pre-existing failure-path tests.
• cargo clippy -p mlxcel --lib --tests -- -D warnings, the exact command the CI clippy job runs, is clean. cargo fmt --check is clean.