Fix --wait-for-child-to-exit not working with overlay windows

The issue was a closure problem where the on_child_death callback
referenced a responder variable that was created after do_launch()
was called. This caused the callback to fail when the child process
exited because responder didn't exist in the callback's closure.

The fix creates the responder before defining the callback and adds
a null check in the callback to ensure it exists before using it.

This allows commands like:
  kitten @ launch --type=overlay --wait-for-child-to-exit bash -c "echo test"
to properly wait for the child process to exit and return the exit code.

kitty/rc/launch.py

@@ -132,6 +132,11 @@ instead of the active tab
         if tabs and tabs[0]:
             target_tab = tabs[0]
 
+        # Create responder before defining callback to avoid closure issue
+        responder = None
+        if payload_get('wait_for_child_to_exit') and not payload_get('no_response'):
+            responder = self.create_async_responder(payload_get, window)
+
         def on_child_death(exit_status: int, exc: Exception | None) -> None:
             code = os.waitstatus_to_exitcode(exit_status)
             ans = str(code)
@@ -141,7 +146,8 @@ instead of the active tab
                     ans = Signals(-code).name
                 except ValueError:
                     pass
-            responder.send_data(ans)
+            if responder is not None:
+                responder.send_data(ans)
 
         w = do_launch(
             boss, opts, payload_get('args') or [], target_tab=target_tab, rc_from_window=window, base_env=base_env,
@@ -152,7 +158,6 @@ instead of the active tab
         if not payload_get('wait_for_child_to_exit'):
             return str(0 if w is None else w.id)
 
-        responder = self.create_async_responder(payload_get, window)
         return AsyncResponse()
 
     def cancel_async_request(self, boss: 'Boss', window: Window | None, payload_get: PayloadGetType) -> None: