Support paginated thread forks (#35220)

## Why

`thread/fork` rejected threads using paginated history, so they could not use the same fork workflow as legacy threads.

## What changed

- Create paginated forks by referencing a frozen source-history prefix while persisting only child-owned records in the new rollout.
- Support latest, `lastTurnId`, and `beforeTurnId` boundaries, including inherited lineage, model-context reconstruction, interruption markers, and approval-reviewer inheritance.
- Make paginated reads and occurrence search traverse referenced history while excluding source records added after the fork.
- Coordinate fork preparation with archive and delete operations, and materialize compressed rollouts before they become fork references.

## Testing

Added app-server and thread-store coverage for boundary selection, active turns, inherited reads and search, compressed lineage, and concurrent source lifecycle operations.

GitOrigin-RevId: 66052d69517d7fb97a3b7817ab77a39c5ad995fd

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

@@ -30,6 +30,7 @@ pub use types::CreateThreadParams;
 pub use types::DeleteThreadParams;
 pub use types::DeleteThreadsParams;
 pub use types::ExtraConfig;
+pub use types::ForkBoundary;
 pub use types::GitInfoPatch;
 pub use types::ItemPage;
 pub use types::ItemSortKey;
@@ -37,6 +38,8 @@ pub use types::ListItemsParams;
 pub use types::ListThreadsParams;
 pub use types::ListTurnsParams;
 pub use types::LoadThreadHistoryParams;
+pub use types::PrepareForkParams;
+pub use types::PreparedFork;
 pub use types::ReadThreadByRolloutPathParams;
 pub use types::ReadThreadParams;
 pub use types::ResumeThreadParams;