Add goal extension idle continuation (#25060)

## Why

The goal extension needs a way to resume an active goal after the thread
becomes idle, but the old core goal runtime should not be refactored as
part of this step. The missing piece is a small core-owned turn-start
primitive: let an extension ask for a normal model turn only when the
thread is idle, and otherwise fail without injecting into whatever is
currently active.

## What Changed

- Adds `CodexThread::try_start_turn_if_idle(...)` as the narrow
extension-facing primitive for synthetic idle work.
- Implements the session side so it refuses to start when:
  - the provided input is empty,
  - the session is in plan mode,
  - a turn is already active, or
  - trigger-turn mailbox work is pending.
- Gives trigger-turn mailbox work priority if it appears while the idle
turn is being prepared.
- Wires `GoalExtension::on_thread_idle` to read the active persisted
goal and submit the continuation prompt through this idle-only
primitive.
- Keeps the legacy core goal continuation implementation in place
instead of folding it into this PR.

## Behavior

This is intentionally best-effort. If `try_start_turn_if_idle` observes
that the thread is not idle, or that higher-priority mailbox work should
run first, it returns the input to the caller. The goal extension drops
that continuation prompt and waits for a future idle opportunity instead
of injecting stale synthetic goal text into an active turn.

## Validation

- `just test -p codex-core
try_start_turn_if_idle_rejects_active_turn_without_injecting`
- `just test -p codex-goal-extension`

codex-rs/core/src/codex_thread.rs

@@ -278,6 +278,14 @@ impl CodexThread {
         self.codex.session.inject_if_running(items).await
     }
 
+    /// Starts a regular turn with model-visible items only if the thread is idle.
+    pub async fn try_start_turn_if_idle(
+        &self,
+        items: Vec<ResponseItem>,
+    ) -> Result<(), Vec<ResponseItem>> {
+        self.codex.session.try_start_turn_if_idle(items).await
+    }
+
     pub async fn set_app_server_client_info(
         &self,
         app_server_client_name: Option<String>,