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/app-server-protocol/src/protocol/v2/thread.rs

@@ -501,6 +501,12 @@ pub struct ThreadForkParams {
     #[ts(optional = nullable)]
     pub last_turn_id: Option<String>,
 
+    /// Optional turn id to fork before, excluding that turn and all later turns.
+    /// Cannot be combined with `last_turn_id`.
+    #[experimental("thread/fork.beforeTurnId")]
+    #[ts(optional = nullable)]
+    pub before_turn_id: Option<String>,
+
     /// [UNSTABLE] Specify the rollout path to fork from.
     /// If specified, the thread_id param will be ignored.
     #[experimental("thread/fork.path")]
@@ -561,6 +567,12 @@ pub struct ThreadForkParams {
     #[experimental("thread/fork.excludeTurns")]
     #[serde(default, skip_serializing_if = "std::ops::Not::not")]
     pub exclude_turns: bool,
+    /// When true, carry the source thread's current goal into the fork without
+    /// starting its initial automatic continuation. The next explicit turn owns
+    /// the goal lifecycle, and normal automatic continuation resumes after it.
+    #[experimental("thread/fork.deferGoalContinuation")]
+    #[serde(default, skip_serializing_if = "std::ops::Not::not")]
+    pub defer_goal_continuation: bool,
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS, ExperimentalApi)]