Branch conversations when editing earlier TUI prompts (#33201)

## What changed

- Fork the conversation immediately before the selected prompt instead of rolling back the current thread in place, preserving the original conversation.
- Reopen the selected prompt in the new conversation with its images, text elements, and mention bindings intact.
- Start a fresh conversation when editing the first prompt, show a dedicated continuation notice, and restore the prompt with an error if branching fails.

## Testing

- Cover branching before first and later prompts, source preservation, prompt restoration, hidden review prompts, mid-turn steering, in-progress turns, stale transcript entries, and mention bindings.

GitOrigin-RevId: 15473535b3f9733b0300711a498969e6d389db79

codex-rs/tui/src/app_event.rs

@@ -240,6 +240,13 @@ pub(crate) enum AppEvent {
     /// Fork the current session into a new thread.
     ForkCurrentSession,
 
+    /// Branch before a selected prompt and reopen it in the new thread's composer.
+    ForkSessionForPromptEdit {
+        thread_id: ThreadId,
+        nth_user_message: usize,
+        prompt: UserMessage,
+    },
+
     /// Request to exit the application.
     ///
     /// Use `ShutdownFirst` for user-initiated quits so core cleanup runs and the
@@ -691,15 +698,6 @@ pub(crate) enum AppEvent {
     /// finalization.
     ConsolidateProposedPlan(String),
 
-    /// Apply rollback semantics to local transcript cells.
-    ///
-    /// This is emitted when rollback was not initiated by the current
-    /// backtrack flow so trimming occurs in AppEvent queue order relative to
-    /// inserted history cells.
-    ApplyThreadRollback {
-        num_turns: u32,
-    },
-
     StartCommitAnimation,
     StopCommitAnimation,
     CommitTick,