content test: Add explicit test case for Zulip Cloud external image preview

This kind of URL is already covered in the tests of *clusters* of
image previews, but it seems helpful to isolate it like this with
its own comment.

We're also about to add a third variant, testing that we accept
images generated with an arbitrary CAMO_URI, which actually we don't
currently do, so that'll be a regression test.

Author: chrisbobbe

test/model/content_test.dart

@@ -785,8 +785,8 @@ class ContentExample {
     ]),
   ]);
 
-  static const imagePreviewSingleExternal = ContentExample(
-    'single image preview external',
+  static const imagePreviewSingleExternal1 = ContentExample(
+    'single image preview external, src starts with /external_content',
     // https://chat.zulip.org/#narrow/stream/7-test-here/topic/Greg/near/1892172
     "https://upload.wikimedia.org/wikipedia/commons/7/78/Verregende_bloem_van_een_Helenium_%27El_Dorado%27._22-07-2023._%28d.j.b%29.jpg",
     '<div class="message_inline_image">'
@@ -799,6 +799,21 @@ class ContentExample {
     ]),
   ]);
 
+  static const imagePreviewSingleExternal2 = ContentExample(
+    'single image preview external, src starts with https://uploads.zulipusercontent.net/',
+    // Zulip Cloud has CAMO_URI = "https://uploads.zulipusercontent.net/";
+    // this example is from a DM on a closed Zulip Cloud org.
+    "https://upload.wikimedia.org/wikipedia/commons/7/78/Verregende_bloem_van_een_Helenium_%27El_Dorado%27._22-07-2023._%28d.j.b%29.jpg",
+    '<div class="message_inline_image">'
+      '<a href="https://upload.wikimedia.org/wikipedia/commons/7/78/Verregende_bloem_van_een_Helenium_%27El_Dorado%27._22-07-2023._%28d.j.b%29.jpg">'
+      '<img src="https://uploads.zulipusercontent.net/99742b0f992be15283c428dd42f3b9f5db138d69/68747470733a2f2f75706c6f61642e77696b696d656469612e6f72672f77696b6970656469612f636f6d6d6f6e732f372f37382f566572726567656e64655f626c6f656d5f76616e5f65656e5f48656c656e69756d5f253237456c5f446f7261646f2532372e5f32322d30372d323032332e5f253238642e6a2e622532392e6a7067"></a></div>', [
+    ImagePreviewNodeList([
+      ImagePreviewNode(srcUrl: 'https://uploads.zulipusercontent.net/99742b0f992be15283c428dd42f3b9f5db138d69/68747470733a2f2f75706c6f61642e77696b696d656469612e6f72672f77696b6970656469612f636f6d6d6f6e732f372f37382f566572726567656e64655f626c6f656d5f76616e5f65656e5f48656c656e69756d5f253237456c5f446f7261646f2532372e5f32322d30372d323032332e5f253238642e6a2e622532392e6a7067',
+        thumbnailUrl: null, loading: false,
+        originalWidth: null, originalHeight: null),
+    ]),
+  ]);
+
   static const imagePreviewInvalidUrl = ContentExample(
     'single image preview with invalid URL',
     null, // hypothetical, to test for a risk of crashing
@@ -1816,7 +1831,8 @@ void main() async {
   testParseExample(ContentExample.imagePreviewSingleNoDimensions);
   testParseExample(ContentExample.imagePreviewSingleNoThumbnail);
   testParseExample(ContentExample.imagePreviewSingleLoadingPlaceholder);
-  testParseExample(ContentExample.imagePreviewSingleExternal);
+  testParseExample(ContentExample.imagePreviewSingleExternal1);
+  testParseExample(ContentExample.imagePreviewSingleExternal2);
   testParseExample(ContentExample.imagePreviewInvalidUrl);
   testParseExample(ContentExample.imagePreviewCluster);
   testParseExample(ContentExample.imagePreviewClusterNoThumbnails);