(PC-23699)[API] test: update serialization decorator test

I specifically commit that separately because I am not 100% confident this actually tests the same thing

api/tests/serialization/serialization_decorator_test.py

@@ -3,9 +3,7 @@
 from unittest.mock import patch
 
 from flask.blueprints import Blueprint
-import pytest
 
-from pcapi.models import api_errors
 from pcapi.routes.serialization import BaseModel
 from pcapi.serialization.decorator import spectree_serialize
 
@@ -42,6 +40,12 @@ def spectree_post_test_endpoint():
     endpoint_method()
 
 
+@test_blueprint.route("/validation", methods=["POST"])
+@spectree_serialize(on_success_status=206)
+def spectree_empty_form_validation(form: TestQueryModel):
+    return
+
+
 @test_bookings_blueprint.route("/bookings", methods=["GET"])
 @spectree_serialize(on_success_status=204)
 def spectree_get_booking_test_endpoint():
@@ -143,12 +147,12 @@ def test_post_with_content_type_with_invalid_body_throw_error(self, client, capl
         )
         assert response.status_code == 400
 
-    def test_http_form_validation(self):
-        @spectree_serialize(response_model=TestResponseModel, on_success_status=206)
-        def mock_func(form: TestQueryModel):
-            return
-
-        with pytest.raises(api_errors.ApiErrors) as exc_info:
-            mock_func(form=None)
+    def test_http_form_validation(self, client):
+        response = client.post(
+            "/test-blueprint/validation", form=None, headers={"Content-Type": "application/x-www-form-urlencoded"}
+        )
 
-        assert exc_info.value.errors == {"compulsory_int_query": "field required"}
+        assert response.status_code == 400
+        assert response.json == {
+            "compulsory_int_query": ["Ce champ est obligatoire"],
+        }