fix(webhook): widen INSECURE_NO_AUTH loopback check + tests + docs

Follow-up to the previous commit:
- Add _is_loopback_host() helper covering 127.0.0.1, localhost, ::1,
  ip6-localhost, ip6-loopback (case-insensitive). Empty/None host is
  treated as non-loopback since unset usually means public default bind.
- Fix mixed-indent comment in the safety rail (comment now aligned with
  the if-block) and collapse the nested-if into one condition.
- Add TestInsecureNoAuthSafetyRail covering rejection on 0.0.0.0, a LAN
  IP, and empty host; allowance on 127.0.0.1/localhost; plus unit-level
  parametrized coverage of _is_loopback_host for spellings we can't bind
  in the hermetic test env (::1, ip6-localhost, ip6-loopback).
- Pin test_connect_starts_server + test_webhook_deliver_only defaults
  to 127.0.0.1 so they keep passing under the new rail.
- Document the behavior in website/docs/user-guide/messaging/webhooks.md.

tests/gateway/test_webhook_deliver_only.py

@@ -33,7 +33,7 @@ from gateway.platforms.webhook import WebhookAdapter, _INSECURE_NO_AUTH
 # ---------------------------------------------------------------------------
 
 def _make_adapter(routes, **extra_kw) -> WebhookAdapter:
-    extra = {"host": "0.0.0.0", "port": 0, "routes": routes}
+    extra = {"host": "127.0.0.1", "port": 0, "routes": routes}
     extra.update(extra_kw)
     config = PlatformConfig(enabled=True, extra=extra)
     return WebhookAdapter(config)