test: measure dots TTS PCM quality at its native 48 kHz rate

The response preserves the native sample rate when none is requested. The HNR helper assumed 24 kHz, halving the perceived pitch of dots.tts-soar output and using the wrong autocorrelation window. Let PCM tests declare the native rate, honor requested resampling rates first, and retain the quality thresholds. Cover the failure with a 120 Hz synthetic voice and rate-selection tests.

Signed-off-by: tzhouam <tzhouam@connect.ust.hk>

tests/e2e/online_serving/test_dots_tts_expansion.py

@@ -85,5 +85,7 @@ def test_text_only_nonstreaming_pcm_001(omni_server, openai_client) -> None:
         "voice": "default",
         "min_audio_bytes": _MIN_AUDIO_BYTES,
         "min_hnr_db": -2.0,
+        # dots.tts-soar emits native 48 kHz PCM, which has no rate header.
+        "expected_sample_rate": 48_000,
     }
     openai_client.send_audio_speech_request(request_config)