## Why
Long realtime conversations could lose speech when active transcripts and handoff transcript fields were truncated.
## What changed
- Record completed V3 transcript chunks in model and client history once the active transcript exceeds 4 KiB, without triggering inference.
- Keep unfinished utterances pending to preserve conversation order, and persist completed chunks before subsequent handoffs.
- Preserve full V3 transcript text at handoff and session-end tail flush, and avoid adding handoff input already recorded in transcript history.
## Testing
Add integration coverage for long transcripts, XML escaping, recording before handoff without inference, repeated handoff input, and both session-end tail flush settings.
GitOrigin-RevId: 62218b811809c4cf5b16fb0247a335eb474f9ab9
guinness-oai
c9750d7f50