Preserve child waiters during process termination (#37498)

## Why

Aborting the child waiter during termination can leave an exited PTY child
unreaped and prevent the session from recording its exit status.

## What changed

- Detach the child waiter instead of aborting it when terminating or dropping a
  `ProcessHandle`, while continuing to abort the I/O helper tasks.
- Cover explicit termination and handle drops for pipe processes, plus queued
  PTY waiters and PTY process-group termination on Unix.

GitOrigin-RevId: 313787f4edec80b30cf5a78f0cc6733f1520d9a4

codex-rs/utils/pty/src/process.rs

@@ -242,7 +242,7 @@ impl ProcessHandle {
         result
     }
 
-    /// Attempts to kill the child and abort helper tasks.
+    /// Attempts to kill the child and abort I/O helper tasks.
     pub fn terminate(&self) {
         self.request_terminate();
 
@@ -264,7 +264,8 @@ impl ProcessHandle {
         if let Ok(mut h) = self.wait_handle.lock()
             && let Some(handle) = h.take()
         {
-            handle.abort();
+            // Even a queued PTY waiter must run to reap the terminated child.
+            drop(handle);
         }
     }
 }