Accept user input when starting idle turns (#36947)

## What changed

- Generalize `CodexThread::try_start_turn_if_idle` to accept exported
  `TurnInput` values, preserving user content and client IDs in the first model
  request.
- Allow non-empty user input to start an idle turn in Plan mode while retaining
  the Plan-mode rejection for automatic response items and empty user input.
- Process idle user input through the normal user-turn path so connector
  selections are cleared, prompt telemetry is recorded, and referenced skill
  instructions are included.

## Testing

- Cover idle user input in Default and Plan modes, response-item mailbox
  merging, empty-input rejection, and skill instruction injection.

GitOrigin-RevId: 116667d160d4464b525d9e4c7e8cc05d23a94008

codex-rs/ext/goal/src/runtime.rs

@@ -4,6 +4,7 @@ use std::sync::atomic::AtomicBool;
 use std::sync::atomic::Ordering;
 
 use codex_core::ThreadManager;
+use codex_core::TurnInput;
 use codex_protocol::ThreadId;
 use codex_protocol::models::ResponseItem;
 use codex_protocol::protocol::ThreadGoal;
@@ -402,7 +403,10 @@ impl GoalRuntimeHandle {
         }
         let item = continuation_steering_item(&protocol_goal_from_state(goal));
 
-        if let Err(err) = thread.try_start_turn_if_idle(vec![item]).await {
+        if let Err(err) = thread
+            .try_start_turn_if_idle(vec![TurnInput::ResponseItem(item)])
+            .await
+        {
             let reason = err.reason();
             tracing::debug!(
                 ?reason,