Fix stale thread-name resume lookups (#16646)

Addresses #15943

Problem: Name-based resume could stop on a newer session_index entry
whose rollout was never persisted, shadowing an older saved thread with
the same name.

Solution: Materialize rollouts before indexing thread names and make
name lookup skip unresolved entries until it finds a persisted rollout.

codex-rs/core/src/codex_tests.rs

@@ -2355,7 +2355,7 @@ async fn attach_rollout_recorder(session: &Arc<Session>) -> PathBuf {
     let recorder = RolloutRecorder::new(
         config.as_ref(),
         RolloutRecorderParams::new(
-            ThreadId::default(),
+            session.conversation_id,
             /*forked_from_id*/ None,
             SessionSource::Exec,
             BaseInstructions::default(),