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/src/request_processors.rs

@@ -330,6 +330,7 @@ use codex_core::path_utils;
 #[cfg(test)]
 use codex_core::read_head_for_summary;
 use codex_core::sandboxing::SandboxPermissions;
+use codex_core::truncate_rollout_after_turn_id;
 use codex_core::windows_sandbox::WindowsSandboxLevelExt;
 use codex_core::windows_sandbox::WindowsSandboxSetupMode as CoreWindowsSandboxSetupMode;
 use codex_core::windows_sandbox::WindowsSandboxSetupRequest;

codex-rs/app-server/src/request_processors/thread_processor.rs

@@ -3387,6 +3387,7 @@ impl ThreadRequestProcessor {
     ) -> Result<(), JSONRPCErrorError> {
         let ThreadForkParams {
             thread_id,
+            last_turn_id,
             path,
             model,
             model_provider,
@@ -3421,12 +3422,20 @@ impl ThreadRequestProcessor {
         let history_items = source_thread
             .history
             .take()
-            .map(|history| Arc::new(history.items))
+            .map(|history| history.items)
             .ok_or_else(|| {
                 internal_error(format!(
                     "thread {source_thread_id} did not include persisted history"
                 ))
             })?;
+        let history_items = if let Some(last_turn_id) = last_turn_id.as_deref() {
+            Arc::new(
+                truncate_rollout_after_turn_id(&history_items, last_turn_id)
+                    .map_err(|err| core_thread_write_error("truncate thread for fork", err))?,
+            )
+        } else {
+            Arc::new(history_items)
+        };
         let history_cwd = Some(source_thread.cwd.clone());
 
         // Persist Windows sandbox mode.