✨(mailboxes) manage bad secret sent to dimail API

- manage 403 returned by dimail API when mail domain secret is not valid
- improve some tests
- improve MailboxFactory to mock success for dimail API POST call
- override 403.html to return a nice failing error in django admin
- an error message is displayed on mailbox creation form of frontend

src/backend/mailbox_manager/factories.py

@@ -2,10 +2,14 @@
 Mailbox manager application factories
 """
 
+import re
+
 from django.utils.text import slugify
 
 import factory.fuzzy
+import responses
 from faker import Faker
+from rest_framework import status
 
 from core import factories as core_factories
 from core import models as core_models
@@ -76,3 +80,35 @@ class MailboxFactory(factory.django.DjangoModelFactory):
     )
     domain = factory.SubFactory(MailDomainEnabledFactory)
     secondary_email = factory.Faker("email")
+
+    @classmethod
+    def _create(cls, model_class, *args, use_mock=True, **kwargs):
+        domain = kwargs["domain"]
+        if use_mock and isinstance(domain, models.MailDomain):
+            with responses.RequestsMock() as rsps:
+                # Ensure successful response using "responses":
+                rsps.add(
+                    rsps.GET,
+                    re.compile(r".*/token/"),
+                    body='{"access_token": "domain_owner_token"}',
+                    status=status.HTTP_200_OK,
+                    content_type="application/json",
+                )
+                rsps.add(
+                    rsps.POST,
+                    re.compile(rf".*/domains/{domain.name}/mailboxes/"),
+                    body=str(
+                        {
+                            "email": f"{kwargs['local_part']}@{domain.name}",
+                            "password": "newpass",
+                            "uuid": "uuid",
+                        }
+                    ),
+                    status=status.HTTP_201_CREATED,
+                    content_type="application/json",
+                )
+
+                result = super()._create(model_class, *args, **kwargs)
+        else:
+            result = super()._create(model_class, *args, **kwargs)
+        return result