Migrate fork and resume reads to thread store (#18900)

- Route cold thread/resume and thread/fork source loading through
ThreadStore reads instead of direct rollout path operations
- Keep lookups that explicitly specify a rollout-path using the local
thread store methods but return an invalid-request error for remote
ThreadStore configurations
- Add some additional unit tests for code path coverage

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

@@ -329,7 +329,7 @@ impl Session {
                             Arc::clone(&thread_store),
                             ResumeThreadParams {
                                 thread_id: resumed_history.conversation_id,
-                                rollout_path: Some(resumed_history.rollout_path.clone()),
+                                rollout_path: resumed_history.rollout_path.clone(),
                                 history: Some(resumed_history.history.clone()),
                                 include_archived: true,
                                 event_persistence_mode,