test_docs: Add test to check all images in integration docs.

Instead of a separate `test_integration_doc_images` function, the test
is executed as part of `test_integration_doc_endpoints`, to optimize
performance by re-using the doc response.

zerver/tests/test_docs.py

@@ -321,7 +321,9 @@ class DocPageTest(ZulipTestCase):
         )
 
     def test_integration_doc_endpoints(self) -> None:
-        self._test(
+        images_in_docs = set()
+
+        response = self._test(
             "/integrations/",
             expected_strings=[
                 "native integrations.",
@@ -330,10 +332,16 @@ class DocPageTest(ZulipTestCase):
                 "IFTTT",
             ],
         )
+        page = response.content.decode("utf-8")
+        for image in re.findall(r"/static/images/integrations/(logos/.*)\"", page):
+            images_in_docs.add(image)
 
         for integration in INTEGRATIONS:
             url = f"/integrations/doc-html/{integration}"
-            self._test(url, expected_strings=[])
+            response = self._test(url, expected_strings=[])
+            doc = response.content.decode("utf-8")
+            for image in re.findall(r"/static/images/integrations/(.*)\"", doc):
+                images_in_docs.add(image)
 
         result = self.client_get(
             "/integrations/doc-html/nonexistent_integration",
@@ -342,6 +350,32 @@ class DocPageTest(ZulipTestCase):
         )
         self.assertEqual(result.status_code, 404)
 
+        directory = "static/images/integrations"
+        images_in_dir = {
+            image_path
+            for root, _, files in os.walk(directory)
+            for file in files
+            if "bot_avatars"
+            not in (image_path := os.path.relpath(os.path.join(root, file), directory))
+        }
+
+        self.assertEqual(
+            images_in_dir,
+            images_in_docs,
+            (
+                "\n\nThe following images are not used in documentation and can be removed:\n"
+                + "\n".join(extra_images)
+                if (extra_images := images_in_dir - images_in_docs)
+                else ""
+            )
+            + (
+                "\n\nThe following images are used in documentation but do not exist:\n"
+                + "\n".join(missing_images)
+                if (missing_images := images_in_docs - images_in_dir)
+                else ""
+            ),
+        )
+
     def test_integration_pages_open_graph_metadata(self) -> None:
         og_description = '<meta property="og:description" content="Zulip comes with over'