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/rollout/src/recorder.rs

@@ -913,7 +913,7 @@ impl RolloutRecorder {
         Ok(InitialHistory::Resumed(ResumedHistory {
             conversation_id,
             history: items,
-            rollout_path: path.to_path_buf(),
+            rollout_path: Some(path.to_path_buf()),
         }))
     }