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/thread-store/src/lib.rs

@@ -25,6 +25,7 @@ pub use types::GitInfoPatch;
 pub use types::ListThreadsParams;
 pub use types::LoadThreadHistoryParams;
 pub use types::OptionalStringPatch;
+pub use types::ReadThreadByRolloutPathParams;
 pub use types::ReadThreadParams;
 pub use types::ResumeThreadParams;
 pub use types::SortDirection;