[Test] Add e2e tests for Qwen3-TTS speech endpoint

[linyueqian]

PLEASE FILL IN THE PR DESCRIPTION HERE ENSURING ALL CHECKLIST ITEMS (AT THE BOTTOM) HAVE BEEN CONSIDERED.

Purpose

Add e2e tests for Qwen3-TTS /v1/audio/speech endpoint. The existing unit tests used mocks that didn't match real behavior, allowing bugs like #1159 to slip through undetected.

Also add doc for tts.

Test Plan

pytest tests/e2e/online_serving/test_qwen3_tts.py -v -s

Test Result

Verified the test correctly catches the multimodal_output bug:

• Without fix (main branch):
  FAILED tests/e2e/online_serving/test_qwen3_tts.py::TestQwen3TTSCustomVoice::test_speech_english_basic
  Server returns {"error":{"message":"TTS model did not produce audio output."}} instead of WAV audio.

• With fix:
  PASSED - Valid WAV audio returned (87KB, 24000Hz mono)

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
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[hsliuustc0106]

could you please also add the api server endpoint in doc?

[Copilot]

Pull request overview

This PR adds comprehensive e2e tests for the Qwen3-TTS model's /v1/audio/speech endpoint. The tests were created in response to bug #1159, where unit tests with mocks didn't catch real behavior issues. The new tests verify actual model inference without mocks.

Changes:

• Adds e2e tests for Qwen3-TTS CustomVoice and VoiceDesign models
• Tests /v1/audio/speech, /v1/audio/voices, and /v1/models endpoints
• Includes regression test for multimodal_output bug that caused "TTS model did not produce audio output" error

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[linyueqian]

could you please also add the api server endpoint in doc?

OK. I have added that doc.

[congw729]

Please add markers as suggested; more usage could be checked at #577

[linyueqian]

Please add markers as suggested; more usage could be checked at #577

Thank you! I have added them.

[hsliuustc0106]

fix ci please

[linyueqian]

fix ci please

Need to merge #1203 first. The test catches that bug so it's expected to fail without it.