feat(app-server): add optional turn_id to thread/fork (#30277)

## Description

This adds stable optional `turnId` support to `thread/fork`. When
supplied, the fork copies persisted history through that terminal turn,
inclusive, and drops later turns from the new thread.

Omitting or passing `null` preserves the existing full-history fork
behavior, including the interruption marker when the stored source
history ends mid-turn.

## Why

We're deprecating `thread/rollback` and this will help certain UX use
cases work around it by using `thread/fork` + `turn_id` instead.

codex-rs/app-server-protocol/src/protocol/v2/thread.rs

@@ -494,6 +494,13 @@ impl From<ThreadTurnsListResponse> for TurnsPage {
 pub struct ThreadForkParams {
     pub thread_id: String,
 
+    /// Optional last turn id to fork through, inclusive.
+    ///
+    /// When specified, turns after `last_turn_id` are omitted from the fork.
+    /// The referenced turn cannot be in progress.
+    #[ts(optional = nullable)]
+    pub last_turn_id: Option<String>,
+
     /// [UNSTABLE] Specify the rollout path to fork from.
     /// If specified, the thread_id param will be ignored.
     #[experimental("thread/fork.path")]