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/functionaltests.c

@@ -8,6 +8,12 @@
  * init_prof_test (starts stabber server and profanity client)
  * and close_prof_test (cleanup) as setup/teardown functions.
  *
+ * NOTE: We restart client and server for each test to ensure complete
+ * isolation. This prevents state leakage between tests (roster entries,
+ * MUC rooms, presence subscriptions, etc.). While slower, it eliminates
+ * flaky tests caused by leftover state. The overhead is acceptable since
+ * functional tests run less frequently than unit tests.
+ *
  * Tests are organized into groups for better maintainability:
  *   Group 1: Connection, Ping, Rooms, Presence
  *   Group 2: Messages, Receipts, Roster management