refactor(tests): migrate to content-based stubbing

- Replace stbbr_for_id() with stbbr_for_query()/stbbr_send()
- Content-based stubbing matches stanzas by namespace instead of ID
- Use regex assertions for flexible output matching
- Fix timing issues in chat_session and presence tests

tests/functionaltests/test_message.c

@@ -4,7 +4,6 @@
 #include <string.h>
 
 #include <stabber.h>
-#include <expect.h>
 
 #include "proftest.h"