Preserve thread context when retrying or editing turns (#33211)

## What changed

- Add experimental `beforeTurnId` support to `thread/fork` for copying history strictly before a turn, including an in-progress turn.
- Use before-turn forks for TUI backtracking and safety retries so the replacement prompt retains thread context instead of recreating the branch from the preceding turn or a new thread.
- Add experimental `deferGoalContinuation` support so safety-retry forks can inherit an active goal while waiting for the replacement turn before automatic continuation resumes.
- Hide duplicated nested-review prompts when replaying history or selecting a prompt to edit.

## Testing

- Cover before-turn truncation, fork validation, active-goal inheritance, TUI safety retries, backtracking, and nested-review replay.

GitOrigin-RevId: 4915d85ae1929a0b8c40f3ed7dca29c7a5bdcdf3

codex-rs/core/src/lib.rs

@@ -148,6 +148,7 @@ pub use state_db_bridge::StateDbHandle;
 pub use state_db_bridge::init_state_db;
 mod thread_rollout_truncation;
 pub use thread_rollout_truncation::truncate_rollout_after_turn_id;
+pub use thread_rollout_truncation::truncate_rollout_before_turn_id;
 mod tools;
 pub(crate) mod turn_diff_tracker;
 mod turn_metadata;