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_server_session.rs

@@ -85,8 +85,6 @@ use codex_app_server_protocol::ThreadReadParams;
 use codex_app_server_protocol::ThreadReadResponse;
 use codex_app_server_protocol::ThreadResumeParams;
 use codex_app_server_protocol::ThreadResumeResponse;
-use codex_app_server_protocol::ThreadRollbackParams;
-use codex_app_server_protocol::ThreadRollbackResponse;
 use codex_app_server_protocol::ThreadSetNameParams;
 use codex_app_server_protocol::ThreadSetNameResponse;
 use codex_app_server_protocol::ThreadSettingsUpdateParams;
@@ -1114,24 +1112,6 @@ impl AppServerSession {
         Ok(())
     }
 
-    pub(crate) async fn thread_rollback(
-        &mut self,
-        thread_id: ThreadId,
-        num_turns: u32,
-    ) -> Result<ThreadRollbackResponse> {
-        let request_id = self.next_request_id();
-        self.client
-            .request_typed(ClientRequest::ThreadRollback {
-                request_id,
-                params: ThreadRollbackParams {
-                    thread_id: thread_id.to_string(),
-                    num_turns,
-                },
-            })
-            .await
-            .wrap_err("thread/rollback failed in TUI")
-    }
-
     pub(crate) async fn review_start(
         &mut self,
         thread_id: ThreadId,