models: Add assert statement for full_members policy in has_permission.

This commit adds an assert statement in the last block of
has_permission which checks whether the policy_value is
POLICY_FULL_MEMBERS_ONLY. This assert statement is added
for readability.

zerver/models.py

@@ -1454,6 +1454,8 @@ class UserProfile(AbstractBaseUser, PermissionsMixin):
 
         if policy_value == Realm.POLICY_MEMBERS_ONLY:
             return True
+
+        assert policy_value == Realm.POLICY_FULL_MEMBERS_ONLY
         return not self.is_provisional_member
 
     def can_create_streams(self) -> bool: