✅(backend) add test to secure updating user when matched on email

We had doubts that the user was correctly updated in the case where
its identity was matched on the email and not on the sub. I added
a test and confirmed that it was working correctly. I still modified
the backend to update the user based on its "id" instead of its "sub"
because it was confusing, but both actually work the same.

src/backend/core/authentication/backends.py

@@ -134,4 +134,4 @@ class OIDCAuthenticationBackend(MozillaOIDCAuthenticationBackend):
         )
         if has_changed:
             updated_claims = {key: value for key, value in claims.items() if value}
-            self.UserModel.objects.filter(sub=user.sub).update(**updated_claims)
+            self.UserModel.objects.filter(id=user.id).update(**updated_claims)

src/backend/core/tests/authentication/test_backends.py

@@ -130,11 +130,12 @@ def test_authentication_getter_existing_user_with_email(
         ("Jack", "Duy", "jack.duy@example.com"),
     ],
 )
-def test_authentication_getter_existing_user_change_fields(
+def test_authentication_getter_existing_user_change_fields_sub(
     first_name, last_name, email, django_assert_num_queries, monkeypatch
 ):
     """
-    It should update the email or name fields on the user when they change.
+    It should update the email or name fields on the user when they change
+    and the user was identified by its "sub".
     """
     klass = OIDCAuthenticationBackend()
     user = UserFactory(
@@ -164,6 +165,48 @@ def test_authentication_getter_existing_user_change_fields(
     assert user.short_name == first_name
 
 
+@pytest.mark.parametrize(
+    "first_name, last_name, email",
+    [
+        ("Jack", "Doe", "john.doe@example.com"),
+        ("John", "Duy", "john.doe@example.com"),
+    ],
+)
+def test_authentication_getter_existing_user_change_fields_email(
+    first_name, last_name, email, django_assert_num_queries, monkeypatch
+):
+    """
+    It should update the name fields on the user when they change
+    and the user was identified by its "email" as fallback.
+    """
+    klass = OIDCAuthenticationBackend()
+    user = UserFactory(
+        full_name="John Doe", short_name="John", email="john.doe@example.com"
+    )
+
+    def get_userinfo_mocked(*args):
+        return {
+            "sub": "123",
+            "email": user.email,
+            "first_name": first_name,
+            "last_name": last_name,
+        }
+
+    monkeypatch.setattr(OIDCAuthenticationBackend, "get_userinfo", get_userinfo_mocked)
+
+    # One and only one additional update query when a field has changed
+    with django_assert_num_queries(3):
+        authenticated_user = klass.get_or_create_user(
+            access_token="test-token", id_token=None, payload=None
+        )
+
+    assert user == authenticated_user
+    user.refresh_from_db()
+    assert user.email == email
+    assert user.full_name == f"{first_name:s} {last_name:s}"
+    assert user.short_name == first_name
+
+
 def test_authentication_getter_new_user_no_email(monkeypatch):
     """
     If no user matches the user's info sub, a user should be created.