Prewarm shell snapshots for eligible turns (#42137)

## Why

Shell Snapshot V2 previously captured the login-shell environment lazily when a
command started, adding that work to the command path.

## What changed

- Start one asynchronous snapshot capture after turn hooks accept the turn, so
  eligible local `exec_command` calls can reuse it without blocking the model.
- Use the turn's resolved tools, permissions, sandbox, environment, and shell
  configuration, and skip prewarming for unsupported or network-managed cases.
- Cancel outstanding prewarming during shutdown. Keep failed speculative
  captures retryable by real commands without consuming their retry budget.

## Testing

Added coverage for non-blocking capture and reuse, failure recovery, sandbox and
permission isolation, shutdown cancellation, hook-stopped turns, eligibility
gates, and concurrent capture retries.

GitOrigin-RevId: ff24a4f0328c06653e09d03fd0a3db1eccd65030

codex-rs/core/src/session/turn.rs

@@ -269,6 +269,15 @@ pub(crate) async fn run_turn(
         return Ok(None);
     }
 
+    // Only speculate after hooks accept the turn, using its finalized tools and permissions.
+    {
+        let mut state = sess.state.lock().await;
+        if state.shell_snapshot_prewarm.is_none() {
+            state.shell_snapshot_prewarm =
+                sess.prewarm_shell_snapshots(first_step_context.as_ref());
+        }
+    }
+
     sess.merge_connector_selection(explicitly_enabled_connectors.clone())
         .await;
     sess.set_previous_turn_settings(Some(PreviousTurnSettings {