✨(api) add count mailboxes to MailDomain serializer

Return number of mailboxes of a domain in our API.

src/backend/mailbox_manager/tests/api/mail_domain/test_api_mail_domains_create.py

@@ -114,6 +114,7 @@ def test_api_mail_domains__create_authenticated():
         "created_at": domain.created_at.isoformat().replace("+00:00", "Z"),
         "updated_at": domain.updated_at.isoformat().replace("+00:00", "Z"),
         "abilities": domain.get_abilities(user),
+        "count_mailboxes": 0,
     }
 
     # a new domain with status "pending" is created and authenticated user is the owner
@@ -185,6 +186,7 @@ def test_api_mail_domains__create_authenticated__dimail_failure():
         "created_at": domain.created_at.isoformat().replace("+00:00", "Z"),
         "updated_at": domain.updated_at.isoformat().replace("+00:00", "Z"),
         "abilities": domain.get_abilities(user),
+        "count_mailboxes": 0,
     }
 
     # a new domain with status "failed" is created and authenticated user is the owner

src/backend/mailbox_manager/tests/api/mail_domain/test_api_mail_domains_retrieve.py

@@ -71,6 +71,7 @@ def test_api_mail_domains__retrieve_authenticated_related():
 
     domain = factories.MailDomainEnabledFactory()
     factories.MailDomainAccessFactory(domain=domain, user=user)
+    factories.MailboxFactory.create_batch(10, domain=domain)
 
     response = client.get(
         f"/api/v1.0/mail-domains/{domain.slug}/",
@@ -85,4 +86,5 @@ def test_api_mail_domains__retrieve_authenticated_related():
         "created_at": domain.created_at.isoformat().replace("+00:00", "Z"),
         "updated_at": domain.updated_at.isoformat().replace("+00:00", "Z"),
         "abilities": domain.get_abilities(user),
+        "count_mailboxes": 10,
     }