test(engines): cover hidden MPS compatibility

tests/backend/api/test_engines_route_shape.py

@@ -242,7 +242,10 @@ def test_mps_picker_hides_redundant_omnivoice_sidecar_but_api_accepts_it(
     fresh_app, monkeypatch
 ):
     """Keep stored/direct compatibility ids valid without advertising a duplicate."""
-    from engines.omnivoice_subprocess import OmniVoiceSubprocessBackend
+    from engines.omnivoice_subprocess import (
+        OmniVoiceMPSSubprocessBackend,
+        OmniVoiceSubprocessBackend,
+    )
     from services import tts_backend
 
     _force_host(monkeypatch, "mps")
@@ -264,6 +267,7 @@ def test_mps_picker_hides_redundant_omnivoice_sidecar_but_api_accepts_it(
         row for row in payload["backends"] if row["id"] == "omnivoice"
     )
     assert canonical["isolation_mode"] == "subprocess"
+    assert tts_backend.get_backend_class("omnivoice") is OmniVoiceMPSSubprocessBackend
 
     response = client.post(
         "/engines/select",

tests/test_diagnose.py

@@ -89,7 +89,9 @@ def test_asr_evidence_failure_does_not_drop_tts_evidence(monkeypatch):
     monkeypatch.setattr(
         tts_backend,
         "list_backends",
-        lambda: [{"id": "tts-ok", "available": True, "execution_evidence": evidence}],
+        lambda **_kwargs: [
+            {"id": "tts-ok", "available": True, "execution_evidence": evidence}
+        ],
     )
     monkeypatch.setattr(asr_backend, "active_backend_id", lambda: "asr-broken")
     def fail_asr_registry():