Retry safety-buffered turns on a forked thread (#33207)

## Why

Retrying a safety-buffered turn by rolling back the active thread rewrites the
source conversation and requires the TUI to repair its local transcript state.

## What changed

- Interrupt the buffered turn, then fork after the preceding completed turn and
  submit the prompt on the new thread with the faster model and low reasoning
  effort.
- Start a new thread when the buffered request is the conversation's first turn.
- Preserve committed steering input in the retry, leave the source thread
  intact, and restore the prompt alongside any existing draft if branching
  fails.
- Do not offer the retry action for side conversations or stale and in-progress
  turns.

## Testing

Added coverage for retries with prior context, first-turn retries, committed
steering input, branch failures, existing composer drafts and attachments, and
side conversations.

GitOrigin-RevId: adb49da8236e9212ef0082e033e6f715986b5cd9

codex-rs/tui/src/app_event.rs

@@ -164,12 +164,13 @@ pub(crate) enum AppEvent {
         op: AppCommand,
     },
 
-    /// Interrupt, roll back, and retry a safety-buffered turn with the server-selected model.
+    /// Interrupt, fork, and retry a safety-buffered turn with the server-selected model.
     RetrySafetyBufferedTurn {
         thread_id: ThreadId,
         turn_id: String,
         model: String,
         turn: AppCommand,
+        prompt: UserMessage,
     },
 
     /// Deliver a synthetic history lookup response to a specific thread channel.