mtmd: stop feeding the text stream again during Qwen3-TTS generation (#26706)

mtmd: stop feeding the text stream again during Qwen3-TTS generation (#26706)

The reference implementation has two mutually exclusive prompt layouts.
In non streaming mode the prefill carries the whole utterance text plus
tts_eos summed with codec_pad, and the trailing text hidden collapses to
a single tts_pad row. In streaming mode the prefill carries only the
first text token and the trailing rows stream the rest of the text
followed by tts_eos.

The pipeline built the non streaming prefill but the streaming overlay,
so the talker saw the utterance a second time during generation and read
it twice before emitting codec_eos.

The overlay is now the single tts_pad row that matches the prefill.