fix: slash command message ordering in timeline

[lileilei-camera]

Issue for this PR

Fixes#38613

Type of change

• Bug fix

What does this PR do?

Fix the WebChat slash command message ordering bug where command output appears at the wrong position in the timeline.

Root cause: event-stream race condition — when slash command output messages arrive before the previous turn's final flush events, the sync data array contains messages in the wrong order. Both model.ts and message-timeline.tsx use the unsorted sync array directly.

Fix:

• model.ts: sort messages by time.created in the messages memo (id as tiebreaker), fixing ordering at the data source
• message-timeline.tsx: add orderedMessages memo as defensive layer, passing sorted messages to createTimelineProjection instead of raw sessionMessages

Note: the submission-side race (setBusy/setIdle) has been independently addressed upstream via the serverSync() API refactor.

How did you verify your code works?

1. Rebuilt binary with WebChat frontend embedded
2. Tested on WebChat over multiple days:
   • Slash command output appears in correct chronological order
   • No visible side effects on normal message flow
3. Smoke test: binary launches and responds normally

Screenshots / recordings

Not applicable — behavioral fix, no UI changes.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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