fix(mlx): drain the MLX pool after an in-flight op when unload landed mid-generation

Review follow-up: unload_model() runs inline on the event loop, so when it
lands during a generation it only drops the backend's reference; the
worker's local keeps the model alive and mx.clear_cache() finds nothing to
free. Once the folded load-and-generate (or transcribe) callable finishes
and releases that local, check whether the model was unloaded meanwhile
and drain the pool then. Covered by a test that unloads from inside a
fake model.generate().

backend/tests/test_mlx_unload_clears_cache.py

@@ -80,3 +80,31 @@ def test_mlx_backends_do_not_clear_cache_when_already_unloaded(monkeypatch):
     backend.unload_model()
 
     mock_clear.assert_not_called()
+
+
+@pytest.mark.asyncio
+async def test_mlx_tts_generate_drains_cache_when_unloaded_mid_generation(monkeypatch):
+    """An unload that lands while generate() runs must still release the pool afterwards."""
+    mock_clear = MagicMock()
+    monkeypatch.setattr(mlx_backend, "empty_mlx_cache", mock_clear)
+
+    backend = mlx_backend.MLXTTSBackend(model_size="0.6B")
+
+    class _FakeResult:
+        audio = [0.0, 0.0]
+        sample_rate = 24000
+
+    class _FakeModel:
+        def generate(self, text, **kwargs):
+            # Simulate /models/unload arriving from the event loop mid-generation.
+            backend.unload_model()
+            yield _FakeResult()
+
+    backend.model = _FakeModel()
+    backend._current_model_size = "0.6B"
+
+    audio, sample_rate = await backend.generate("hello", {}, "en")
+
+    assert len(audio) == 2
+    assert backend.model is None
+    mock_clear.assert_called_once()