refactor(tests): improve functional test documentation and cleanup

- Add detailed documentation about test isolation in functionaltests.c
- Improve comments in proftest.c: buffer size explanation, stabber sync TODO
- Add reference to stabber issue #1 for stbbr_wait_stopped() API
- Document prof_output_regex() usage rationale in test_muc.c
- Enhance ping_server test comments for clarity
- Update CONTRIBUTING.md: fix function names (prof_output_regex, prof_timeout)
- Remove redundant commented-out test configurations from ci-build.sh
- Remove obsolete stabber recv suppressions from prof.supp (issue fixed)
- Add clear explanation for disabled assertion in test_chat_session.c
- Add comment explaining /connect max args in cmd_defs.c

tests/functionaltests/test_muc.c

@@ -8,6 +8,14 @@
 
 #include "proftest.h"
 
+/*
+ * NOTE: We use prof_output_regex() throughout this file even for seemingly
+ * exact strings because some output may include timestamps, color codes,
+ * or other dynamic content depending on configuration. Using regex provides
+ * more robust matching. For patterns without regex metacharacters, the
+ * performance difference is negligible.
+ */
+
 void
 sends_room_join(void **state)
 {