* fix(openai-compat): reuse cached engine instances in _resolve_engine
The direct engine-ID path in /v1/audio/speech constructed a fresh
backend per request (return cls()). For SubprocessBackend engines that
meant: a new sidecar process, a full torch import and an engine model
reload on EVERY request (measured ~28s floor per pockettts request on
an M3 Pro), plus another atexit hook registration each time — exactly
what get_engine_instance_for()'s docstring warns against.
Route the explicit-ID path through the same cached-singleton seam the
active-engine path already uses. Unknown/unavailable IDs keep their
400s; tts-1/tts-1-hd and the OmniVoiceBackend special case are
unchanged.
* fix(openai-compat): unload the outgoing engine on explicit-ID switches
Review follow-up (Greptile/CodeRabbit on #1614): caching instances without
a switch rule would let each distinct explicit engine ID stay resident,
accumulating sidecars / multi-GB in-process models. Mirror
get_active_tts_backend's MM2-01 switch rule: a different explicit ID
(omnivoice included, which resolves to the active engine) unloads the
outgoing instance first, best-effort.
* fix(openai-compat): evict via the shared single-engine-resident seam, not a router-local cache
The explicit-ID unload cache (13c14e2c) kept its own instance ref keyed by
model id. The shared engine cache is deliberately keyed by CLASS (registry
rebinds, idle sweeps and engine_memory eviction all mutate it), so the
router's id-keyed ref could go stale and keep serving an instance the
lifecycle system no longer tracked — caught by
test_openai_speech_toggle_off_sends_raw_text in full-suite order, and it
also introduced a novel unload path that ignored the
OMNIVOICE_SINGLE_ENGINE_RESIDENT opt-out.
Drop the router-local cache entirely: _resolve_engine returns the shared
cached singleton (get_engine_instance_for), and create_speech calls
evict_other_tts_engines(backend.id) before warming the engine — the exact
seam /generate uses. That covers every transition (explicit id → explicit
id, explicit id → tts-1/omnivoice aliases), honors the policy opt-out, and
leaves no per-router state to drift. Regression pinned at the route level in
test_speech_request_evicts_other_resident_engines.
* chore(changelog): trim the #1614 entry to the one-liner limit
415 chars against the 400 the style test allows — CI would have failed on it.
---------
Co-authored-by: debpalash <4178343+debpalash@users.noreply.github.com>
