TUI: prompt to implement plan and switch to Execute (#9712)

## Summary
- Replace the plan‑implementation prompt with a standard selection
popup.
- “Yes” submits a user turn in Execute via a dedicated app event to
preserve normal transcript behavior.
- “No” simply dismisses the popup.

<img width="977" height="433" alt="Screenshot 2026-01-22 at 2 00 54 PM"
src="https://github.com/user-attachments/assets/91fad06f-7b7a-4cd8-9051-f28a19b750b2"
/>

## Changes
- Add a plan‑implementation popup using `SelectionViewParams`.
- Add `SubmitUserMessageWithMode` so “Yes” routes through
`submit_user_message` (ensures user history + separator state).
- Track `saw_plan_update_this_turn` so the prompt appears even when only
`update_plan` is emitted.
- Suppress the plan popup on replayed turns, when messages are queued,
or when a rate‑limit prompt is pending.
- Add `execute_mode` helper for collaboration modes.
- Add tests for replay/queued/rate‑limit guards and plan update without
final message.
- Add snapshots for both the default and “No”‑selected popup states.

codex-rs/tui/src/app_event.rs

@@ -240,6 +240,12 @@ pub(crate) enum AppEvent {
     /// Open the custom prompt option from the review popup.
     OpenReviewCustomPrompt,
 
+    /// Submit a user message with an explicit collaboration mode.
+    SubmitUserMessageWithMode {
+        text: String,
+        collaboration_mode: CollaborationMode,
+    },
+
     /// Open the approval popup.
     FullScreenApprovalRequest(ApprovalRequest),