TLDR: use new message phase field emitted by preamble-supported models
to determine whether an AgentMessage is mid-turn commentary. if so,
restore the status indicator afterwards to indicate the turn has not
completed.
### Problem
`commit_tick` hides the status indicator while streaming assistant text.
For preamble-capable models, that text can be commentary mid-turn, so
hiding was correct during streaming but restore timing mattered:
- restoring too aggressively caused jitter/flashing
- not restoring caused indicator to stay hidden before subsequent work
(tool calls, web search, etc.)
### Fix
- Add optional `phase` to `AgentMessageItem` and propagate it from
`ResponseItem::Message`
- Keep indicator hidden during streamed commit ticks, restore only when:
- assistant item completes as `phase=commentary`, and
- stream queues are idle + task is still running.
- Treat `phase=None` as final-answer behavior (no restore) to keep
existing behavior for non-preamble models
### Tests
Add/update tests for:
- no idle-tick restore without commentary completion
- commentary completion restoring status before tool begin
- snapshot coverage for preamble/status behavior
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Co-authored-by: Josh McKinney <joshka@openai.com>
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