Add CI checks for /api-doc/ endpoint (#13210)

* Add CI checks for /api-doc/ endpoint

Ref: https://github.com/inventree/InvenTree/issues/13200

* fix playwright test

* Reduce flakiness

src/backend/InvenTree/InvenTree/test_views.py

@@ -1,6 +1,7 @@
 """Unit tests for the main web views."""
 
 import os
+import re
 
 from django.test import override_settings
 from django.urls import reverse
@@ -18,12 +19,55 @@ class ViewTests(InvenTreeTestCase):
         SITE_URL='http://testserver', CSRF_TRUSTED_ORIGINS=['http://testserver']
     )
     def test_api_doc(self):
-        """Test that the api-doc view works."""
+        """Test that the api-doc view renders a usable Redoc page.
+
+        A 200 response alone is not sufficient - the page is a thin HTML shell
+        which loads Redoc from a CDN and points it at the schema endpoint.
+        Ref: https://github.com/inventree/InvenTree/issues/13200
+        """
         api_url = os.path.join(reverse('index'), 'api-doc') + '/'
 
         response = self.client.get(api_url)
         self.assertEqual(response.status_code, 200)
 
+        html = response.content.decode()
+        schema_url = reverse('schema')
+
+        # The page must point Redoc at our schema endpoint
+        self.assertIn(schema_url, html)
+
+        # Find the Redoc bundle which is loaded by the page
+        scripts = re.findall(r'<script([^>]*)\ssrc="([^"]+)"', html)
+        redoc_scripts = [(attrs, src) for attrs, src in scripts if 'redoc' in src]
+        self.assertEqual(len(redoc_scripts), 1, 'Redoc script not found in api-doc')
+
+        attrs, src = redoc_scripts[0]
+
+        # The Redoc version must be pinned - '@latest' can (and did) change under us
+        match = re.search(r'redoc@(\d+)(?:\.\d+)*/', src)
+        self.assertIsNotNone(
+            match, f'Redoc distribution must be pinned to a version: {src}'
+        )
+
+        # Redoc >= 3 is ES-module only and cannot be loaded via a classic <script>
+        if int(match.group(1)) >= 3:
+            self.assertIn(
+                'type="module"',
+                attrs,
+                f'Redoc {match.group(1)}.x must be loaded as an ES module: {src}',
+            )
+
+        # The schema which Redoc loads must contain generated API data
+        response = self.client.get(schema_url, HTTP_ACCEPT='application/json')
+        self.assertEqual(response.status_code, 200)
+
+        schema = response.json()
+        self.assertTrue(schema['openapi'].startswith('3.'))
+        self.assertEqual(schema['info']['title'], 'InvenTree API')
+        self.assertIn('/api/part/', schema['paths'])
+        self.assertIn('/api/stock/', schema['paths'])
+        self.assertGreater(len(schema['paths']), 100)
+
     @override_settings(
         SITE_URL='http://testserver', CSRF_TRUSTED_ORIGINS=['http://testserver']
     )