fix(tui): undo pending prompts and active sessions - #47343

Summary

Fix /undo in the V2 TUI while a session is running:

• Cancel an undelivered prompt through the existing inbox action, without interrupting the active response.
• For a delivered message, interrupt the session and await session.wait() before staging the revert. Interrupt acknowledgment alone does not mean execution cleanup has settled.
• Restore the prompt to the captured composer only after cancellation or revert succeeds.

Limited to the existing TUI undo handler; no Core/API changes or added tests.

Validation

Validated with OpenCode Drive 2.1.0 against the real TUI and isolated backend, using controlled simulated model responses:

• Pending steer: removed from the inbox, original generation remains active, prompt restored and focused.
• Delivered steer: active generation stopped, correct steering message reverted, prompt restored and focused.
• Active generation without steering: original prompt successfully reverted.
• Idle session: undo and redo both succeed.
• No Session is busy or missing-message error in the successful runs.

Checks: bun typecheck in packages/tui, Prettier check on the changed file, and git diff --check all pass.

Validated head: a0154bcf7c30fcf2326e2dff226ec86c5e56492a (base: 0e143437c88385fa69e233c3084fe1ca55415f70).

Requested by: @nexxeln (nexxel via Slack)