Use StartThreadOptions across thread fork APIs (#44043)

## What changed

Pass `StartThreadOptions` through rollout, loaded-history, and prepared forks instead of separate configuration and startup arguments. Replace `options.initial_history` with the fork snapshot while preserving the other supplied startup options, and update app-server and test callers.

Box the app-server's `thread_fork_inner` future to keep the large fork future out of the shared request dispatcher's stack frame.

GitOrigin-RevId: d54eb8b212dedabaf2c5d99d155637cefd7deb7a

codex-rs/core/src/session/tests.rs

@@ -3708,10 +3708,8 @@ async fn fork_startup_context_then_first_turn_diff_snapshot() -> anyhow::Result<
         .thread_manager
         .fork_thread(
             usize::MAX,
-            fork_config.clone(),
+            core_test_support::test_codex::StartThreadOptions::new(fork_config.clone()),
             rollout_path,
-            /*thread_source*/ None,
-            /*parent_trace*/ None,
         )
         .await?;