Fix Gemma 4 audio processor operation (#752)

## Summary
- emit the registered `Gemma4LogMel` operation for standard Gemma 4
audio preprocessing
- remove the unsupported `type=log_mel` operation attribute
- update the generated-config expectations

## Testing
- Not run (per request).

Author: xiaoyu-work

src/mobius/integrations/ort_genai/auto_export.py

@@ -1384,7 +1384,7 @@ def _write_audio_processor_config(
         proc_filename = "audio_feature_extraction.json"
     elif model_type in _GEMMA4_MODEL_TYPES:
         # Gemma4 USM-style 128-dim log-mel spectrogram via the ort-extensions
-        # ``Gemma4Audio`` op with ``type="log_mel"``.
+        # ``Gemma4LogMel`` op.
         # OrtxCreateSpeechFeatureExtractor requires the feature_extraction.sequence format.
         processor = {
             "feature_extraction": {
@@ -1397,10 +1397,9 @@ def _write_audio_processor_config(
                     },
                     {
                         "operation": {
-                            "name": "gemma4_audio",
-                            "type": "Gemma4Audio",
+                            "name": "gemma4_log_mel",
+                            "type": "Gemma4LogMel",
                             "attrs": {
-                                "type": "log_mel",
                                 "feature_size": 128,
                                 "sampling_rate": 16000,
                                 "frame_length_ms": 20.0,

src/mobius/integrations/ort_genai/auto_export_test.py

@@ -1029,9 +1029,8 @@ def test_audio_gemma4_writes_feature_extraction_json(self, tmp_path):
         seq = data["feature_extraction"]["sequence"]
         assert len(seq) == 2
         assert seq[0]["operation"]["type"] == "AudioDecoder"
-        assert seq[1]["operation"]["type"] == "Gemma4Audio"
+        assert seq[1]["operation"]["type"] == "Gemma4LogMel"
         attrs = seq[1]["operation"]["attrs"]
-        assert attrs["type"] == "log_mel"
         assert attrs["feature_size"] == 128
         assert attrs["sampling_rate"] == 16000
         assert attrs["frame_length_ms"] == 20.0  # noqa: RUF069
@@ -1968,10 +1967,9 @@ class FakeConfig:
         op0 = seq[0]["operation"]
         assert op0["type"] == "AudioDecoder"
 
-        # Second op: Gemma4Audio (type=log_mel) with expected attrs
+        # Second op: Gemma4LogMel with expected attrs
         op1 = seq[1]["operation"]
-        assert op1["type"] == "Gemma4Audio"
-        assert op1["attrs"]["type"] == "log_mel"
+        assert op1["type"] == "Gemma4LogMel"
         assert op1["attrs"]["feature_size"] == 128
         assert op1["attrs"]["sampling_rate"] == 16000
         assert op1["attrs"]["mel_floor"] == 0.001  # noqa: RUF069