content: Relax image-preview parsing, notably to accept arbitrary CAMO_URI

See discussion:
  https://chat.zulip.org/#narrow/channel/412-api-documentation/topic/documenting.20inline.20images/near/2279454

We already test the `src == href` case, as
ContentExample.imagePreviewSingleNoThumbnail, but I added a test
case for an arbitrary CAMO_URI that fails before this commit.

Author: chrisbobbe

lib/model/content.dart

@@ -1413,22 +1413,17 @@ class _ZulipContentParser {
     final String srcUrl;
     final String? thumbnailUrl;
     if (src.startsWith('/user_uploads/thumbnail/')) {
+      // For why we recognize this as the thumbnail form, see discussion:
+      //   https://chat.zulip.org/#narrow/channel/412-api-documentation/topic/documenting.20inline.20images/near/2279872
       srcUrl = href;
       thumbnailUrl = src;
-    } else if (src.startsWith('/external_content/')
-        || src.startsWith('https://uploads.zulipusercontent.net/')) {
-      // This image preview uses camo, which still happens on current servers
-      // (2025-10); discussion:
-      //   https://chat.zulip.org/#narrow/channel/412-api-documentation/topic/documenting.20inline.20images/near/2279235
-      srcUrl = src;
-      thumbnailUrl = null;
-    } else if (href == src)  {
-      // Probably generated by a server before the thumbnailing feature landed:
-      //   https://chat.zulip.org/#narrow/channel/412-api-documentation/topic/documenting.20inline.20images/near/2279234
+    } else {
+      // Known cases this handles:
+      // - `src` starts with CAMO_URI, a server variable (e.g. on Zulip Cloud
+      //   it's "https://uploads.zulipusercontent.net/" in 2025-10).
+      // - `src` matches `href`, e.g. from pre-thumbnailing servers.
       srcUrl = src;
       thumbnailUrl = null;
-    } else {
-      return UnimplementedBlockContentNode(htmlNode: divElement);
     }
 
     double? originalWidth, originalHeight;

test/model/content_test.dart

@@ -814,6 +814,21 @@ class ContentExample {
     ]),
   ]);
 
+  static const imagePreviewSingleExternal3 = ContentExample(
+    'single image preview external, src starts with https://custom.camo-uri.example/',
+    // CAMO_URI (server variable) can be set arbitrarily;
+    // for another possible value, see imagePreviewSingleExternal2.
+    "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://custom.camo-uri.example/99742b0f992be15283c428dd42f3b9f5db138d69/68747470733a2f2f75706c6f61642e77696b696d656469612e6f72672f77696b6970656469612f636f6d6d6f6e732f372f37382f566572726567656e64655f626c6f656d5f76616e5f65656e5f48656c656e69756d5f253237456c5f446f7261646f2532372e5f32322d30372d323032332e5f253238642e6a2e622532392e6a7067"></a></div>', [
+    ImagePreviewNodeList([
+      ImagePreviewNode(srcUrl: 'https://custom.camo-uri.example/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
@@ -1833,6 +1848,7 @@ void main() async {
   testParseExample(ContentExample.imagePreviewSingleLoadingPlaceholder);
   testParseExample(ContentExample.imagePreviewSingleExternal1);
   testParseExample(ContentExample.imagePreviewSingleExternal2);
+  testParseExample(ContentExample.imagePreviewSingleExternal3);
   testParseExample(ContentExample.imagePreviewInvalidUrl);
   testParseExample(ContentExample.imagePreviewCluster);
   testParseExample(ContentExample.imagePreviewClusterNoThumbnails);