Add graceful agent-tree suspension and recovery (#61198)

## Why

Suspending a locally controlled agent tree must preserve unfinished turns and child results that arrive during teardown so they remain available after recovery.

## What changed

- Add `ThreadManager::suspend_agent_tree` to stop durable trees, close their writers, and return unfinished root and named V2 child turn IDs plus any pending root wake.
- Persist terminal child results before suspension succeeds, including results queued for evicted parents. Report failed or unconfirmed writes.
- Add `load_suspended_agent` to restore named V2 descendants through their recorded ancestors without starting model turns.
- Allow empty `continue_turn_if_idle` input to wake on saved results, guarded by the preceding turn ID. Keep mail queued until task registration so suspension can preserve it.

## Testing

Add coverage for completion and wake races, child errors, parent writer failures, evicted parents, descendant restoration, repeated suspension after caller cancellation, and ephemeral roots remaining active.

GitOrigin-RevId: 3041791f51bff2555210f6b2d2443f9f05f68330

codex-rs/core/src/codex_thread.rs

@@ -587,7 +587,9 @@ impl CodexThread {
 
     /// Starts a new internal continuation turn when idle, including in Plan mode.
     /// Rejects if a newer task has started, even if it has already finished.
-    /// The input must be a response item; it is never treated as user authorization.
+    /// Supply an internal response item, or empty input to wake on a result already
+    /// saved in history. An empty wake inherits the preceding turn's attribution
+    /// and adds no message or user authorization.
     pub async fn continue_turn_if_idle(
         &self,
         request: impl Into<WithTurnExtensionData<TurnInputRequest>>,