[CI/Build] Let audio assertions pin the transcript language (#5646)

Signed-off-by: Shenglei Fu <sfu@confluent.io>

tests/helpers/tests/test_assertions.py

@@ -1,9 +1,12 @@
 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
+from types import SimpleNamespace
+
 import pytest
 
-from tests.helpers.assertions import _assert_transcript_matches
+from tests.helpers import assertions
+from tests.helpers.assertions import _assert_transcript_matches, _resolve_audio_transcript
 
 pytestmark = [pytest.mark.core_model, pytest.mark.cpu]
 
@@ -25,3 +28,63 @@ def test_short_transcript_unrelated_text_still_fails():
             expected_text="how are you",
             threshold=0.9,
         )
+
+
+def _capture_transcribe(monkeypatch) -> dict:
+    captured: dict = {}
+
+    def fake_convert(raw_bytes, model_size="small", language=None):
+        captured["model_size"] = model_size
+        captured["language"] = language
+        return "London"
+
+    monkeypatch.setattr(assertions, "convert_audio_bytes_to_text", fake_convert)
+    return captured
+
+
+def test_resolve_transcript_honors_declared_language(monkeypatch):
+    captured = _capture_transcribe(monkeypatch)
+    response = SimpleNamespace(audio_content=None, audio_bytes=b"fake-wav")
+
+    _resolve_audio_transcript(
+        response,
+        {"modalities": ["text", "audio"], "transcript_language": "en"},
+        "full_model",
+        speech_api=False,
+    )
+
+    assert captured["language"] == "en"
+
+
+def test_resolve_transcript_leaves_language_unset_by_default(monkeypatch):
+    captured = _capture_transcribe(monkeypatch)
+    response = SimpleNamespace(audio_content=None, audio_bytes=b"fake-wav")
+
+    _resolve_audio_transcript(
+        response,
+        {"modalities": ["text", "audio"]},
+        "full_model",
+        speech_api=False,
+    )
+
+    assert captured["language"] is None
+    assert captured["model_size"] == "small"
+
+
+def test_escalated_transcript_keeps_declared_language(monkeypatch):
+    # The escalated pass must honour the same language, otherwise a request that
+    # pins one silently falls back to auto-detection on retry.
+    captured = _capture_transcribe(monkeypatch)
+
+    with pytest.raises(AssertionError, match="after ASR escalation"):
+        _assert_transcript_matches(
+            "totally different words",
+            audio_bytes=b"fake-wav",
+            expected_text="how are you",
+            threshold=0.9,
+            escalation_model="large-v3",
+            language="en",
+        )
+
+    assert captured["model_size"] == "large-v3"
+    assert captured["language"] == "en"