chore(tts): adopt the released Graphon TTSChunk protocol

## Summary

Follow-up to #41043. The Dify 1.17 user-visible fix is tracked by #35880 and does not wait for this issue.

## Scope

- Wait for a Graphon release that includes the structured `TTSChunk(data, mime_type)` protocol.
- Upgrade Dify to that released Graphon version.
- Replace the temporary bytes-compatible `TTSAudioChunk` carrier introduced by #41043.
- Remove compatibility code made obsolete by the released protocol.
- Run focused Core, API, SSE, tool, and Web TTS regressions.

## Release relationship

This issue is **not a Dify 1.17 blocker**. Dify 1.17 is tracked by #35880 and implemented by #41043 together with `langgenius/dify-official-plugins#3698`.

The Tongyi WAV declaration and long-text WAV merge remain owned by the companion plugin PR.