[Bugfix][Qwen3-TTS] Handle codec generations that exhaust their token budget (#6728)

Signed-off-by: Sy03 <1370724210@qq.com>

tests/entrypoints/openai_api/test_speech_usage.py

@@ -1,4 +1,5 @@
 # SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM-Omni project
 """Unit tests for speech token-usage accounting (issue #4646).
 
 These exercise the pure usage logic without a model/server: input-token
@@ -205,6 +206,14 @@ def test_output_tokens_from_stage_metrics():
     assert acc.total() == 77
 
 
+def test_output_tokens_preserves_stage0_finish_reason():
+    acc = SpeechOutputTokenCounter()
+    res = _final_res(stage0_out=128)
+    res.metrics["stage_metrics"]["0"]["finish_reason"] = "length"
+    acc.observe(res)
+    assert acc.stage0_finish_reason == "length"
+
+
 def test_output_tokens_takes_max_across_stages():
     acc = SpeechOutputTokenCounter()
     acc.observe(_final_res(stage0_out=42, stage1_out=0))