Preserve viewport edit draft when queued request executes

[liaowang11]

What

When a command is sent (e.g. a queued request executing) while the user is composing in the viewport edit buffer, the draft is now saved to the compose snapshot instead of being wiped by the switch to view mode. The draft is restored on the next return to edit mode.

Why

agent-shell--send-command switches the viewport to view mode and re-initializes the buffer. If a queued request runs while the user is mid-composition in the viewport edit buffer, their in-progress draft was lost. Reusing the existing agent-shell-viewport--compose-snapshot mechanism preserves it.

Checklist

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