Follow-up to cherry-picked PR #6592:
- Extract _webhook_url property to deduplicate URL construction
- Add _find_registered_webhooks() helper for reuse
- Crash resilience: check for existing registration before POSTing
(handles restart after unclean shutdown without creating duplicates)
- Accept 200-299 status range (not just 200) for webhook creation
- Unregister removes ALL matching registrations (cleans up orphaned dupes)
- Add 17 tests covering register/unregister/find/edge cases
**Problem:**
The BlueBubbles iMessage gateway was not receiving incoming messages even though:
1. BlueBubbles Server was properly configured and running
2. Hermes gateway started without errors
3. Webhook listener was started on the configured port
The root cause was that the BlueBubbles adapter only started a local webhook
listener but never registered the webhook URL with the BlueBubbles server via
the API. Without registration, the server doesn't know where to send events.
**Fix:**
1. Added _register_webhook() method that POSTs to /api/v1/webhook with the
listener URL and event types (new-message, updated-message, message)
2. Added _unregister_webhook() method for clean shutdown
3. Both methods handle the case where webhook listens on 0.0.0.0/127.0.0.1
by using 'localhost' as the external hostname
4. Fixed documentation: 'hermes gateway logs' → 'hermes logs gateway'
**API Reference:**
https://docs.bluebubbles.app/server/developer-guides/rest-api-and-webhooks
**Testing:**
- Webhook registration is now automatic when gateway starts
- Failed registration logs a warning but doesn't prevent startup
- Clean shutdown unregisters the webhook
Closes: iMessage gateway not working issue
