fix(tui): prevent model/agent reset on Prompt remount

[dsy122]

Issue for this PR

Closes #34207

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Moving syncedSessionID from component closure to module level. This variable guards against re-syncing model/agent from lastUserMessage() on Prompt remount — when a question or permission dialog appears, the Prompt unmounts and remounts, which previously reset the guard and triggered an unwanted sync, overwriting any manual model selection.

How did you verify your code works?

1. Open session and trigger the agent to trigger question
2. While the agent is still running, switch model from A to B
3. Question appears, and after answering, the model stays B

Screenshots / recordings

2026-06-27.19-54-33.mp4

Checklist

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

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