Route ThreadManager rollout path reads through thread store (#21265)

- Route ThreadManager rollout-path resume/fork through ThreadStore
history reads.
- Add in-memory store coverage proving path-addressed reads are used.

This isn't strictly necessary for the ThreadStore migration, since these
ThreadManager methods _only_ work for path-based lookups, but I'm trying
to migrate all the rollout recorder callsites to use the threadstore
were possible for consistency.

codex-rs/thread-store/src/in_memory.rs

@@ -256,10 +256,16 @@ fn stored_thread_from_state(
         items: history_items.clone(),
     });
     let name = state.names.get(&thread_id).cloned().flatten();
+    let rollout_path = state
+        .rollout_paths
+        .iter()
+        .find_map(|(path, mapped_thread_id)| {
+            (*mapped_thread_id == thread_id).then(|| path.clone())
+        });
 
     Ok(StoredThread {
         thread_id,
-        rollout_path: None,
+        rollout_path,
         forked_from_id: created.forked_from_id,
         preview: String::new(),
         name,