handleGroupDiscovery and handleGroupEnforcement lacked isGroupId()
guards, causing user JIDs (e.g. 1234567890@s.whatsapp.net) to be
incorrectly registered as 'pending' groups in discover mode, and
silently blocked in enforce mode.
Added isGroupId() bail-out at the top of both functions, matching the
existing pattern in ensureGroupActive(). Added 5 regression tests
covering both modes for DMs and preserving correct group behavior.
