fix: resolve transcript-only amd greetings at eot

[chenghao-mou]

Problem: AMD can receive final transcripts without VAD boundaries. Its LLM verdict then remains behind the silence gate until the detection timeout.

Behavior: The existing transcript EOT backstop now supplies the missing silence signal only when no valid VAD end exists. None reuses session STT or LLM models, while omitted values keep cloud defaults. Timeout verdicts retain the transcript.

Fixes #6996
Addresses AGT-3382

Initial prompt and agent context

Model: GPT-5.6

Can you investigate #6996?

Yeah, EOT sounds good.

we could have None means session STT or LLM, so NOT_GIVEN -> default, None -> Inherit, explicit -> explicit.

Yeah, we should preserve the transcript and add tests against the regression cases.

but eot requires VAD too, right?

yes please.

[devin-ai-integration]

✅ Devin Review: No Issues Found

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