docs: update mega-es functional features documentation

Update the functional features document to reflect recent changes in integration logic and new capabilities:

- Update "Last Updated" date to July 25, 2026.
- Clarify that Mega Hub integrations (WhatsApp, Instagram, Messenger) now use dedicated Hub credentials.
- Add Gmail connection verification feature (IMAP/SMTP checks and activity logs).
- Add availability constraints for WhatsApp calls regarding coexistence with the WhatsApp Business App.
- Expand WhatsApp calling features to include call initiation from contact profiles, detailed permission state tracking, and a restriction of one active call per agent.