Add before_request check for request size

The test checks various routes to ensure that request above our
threshold fail before any other processing happens

tests/app/test_request_size.py

@@ -0,0 +1,36 @@
+import pytest
+import json
+
+@pytest.mark.parametrize('endpoint, max_content_length, expected_status_code', [
+    ("/_status", 5*1024*1024, 413),
+    ("/provider-details", 5*1024*1024, 413),
+    ("/v2/notifications/email", 5*1024*1024, 413),
+
+    ("/_status", None, 200),
+    ("/provider-details", None, 405),
+    ("/v2/notifications/email", None, 401),
+])
+def test_request_status_when_content_length_is_set(
+    notify_api,
+    sample_email_template_with_placeholders,
+    mocker,
+    endpoint,
+    max_content_length,
+    expected_status_code):
+
+    notify_api.config['MAX_CONTENT_LENGTH'] = max_content_length
+    large_name = "J" * (max_content_length or 1 + 1)
+    data = {
+        'email_address': 'ok@ok.com',
+        'template_id': str(sample_email_template_with_placeholders.id),
+        'personalisation': {
+            'name': large_name
+        }
+    }
+    with notify_api.test_client() as client:
+        response = client.post(
+            path=endpoint,
+            data=json.dumps(data),
+            headers=[('Content-Type', 'application/json')])
+
+        assert response.status_code == expected_status_code