Clear /ps after /stop (#17416)

Addresses #17311

Problem: `/stop` stops background terminals, but `/ps` can still show
stale entries because the TUI process cache is cleared only after later
exec end events arrive.

Solution: Clear the TUI's tracked unified exec process list and footer
immediately when `/stop` submits background terminal cleanup.

codex-rs/tui/src/chatwidget.rs

@@ -7260,6 +7260,8 @@ impl ChatWidget {
 
     fn clean_background_terminals(&mut self) {
         self.submit_op(AppCommand::clean_background_terminals());
+        self.unified_exec_processes.clear();
+        self.sync_unified_exec_footer();
         self.add_info_message(
             "Stopping all background terminals.".to_string(),
             /*hint*/ None,