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

CONTRIBUTING.md

@@ -167,8 +167,8 @@ Guidelines when writing new functional tests:
 1. Prefer `stbbr_for_query(namespace, xml)` for IQ roster or disco queries where the namespace is stable.
 2. Use `stbbr_send(xml)` for presence, message, and other push style stanzas.
 3. Avoid `stbbr_for_id` unless the protocol flow genuinely requires correlating a specific request/response pair not covered by a namespace query.
-4. Keep assertions tolerant of ordering when possible; rely on `exp_expectl` regex matches rather than hard-coded positions.
-5. If timing flakiness appears, temporarily raise `exp_timeout` around the critical expectation and reset it immediately afterwards.
+4. Keep assertions tolerant of ordering when possible; rely on `prof_output_regex()` matches rather than hard-coded positions.
+5. If timing flakiness appears, temporarily raise `prof_timeout()` around the critical expectation and reset it immediately afterwards.
 
 During the migration it's acceptable for a test file to mix old and new styles; opportunistically refactor brittle sections when touching a test for other reasons.