🐛 Fixed YouTube live embeds failing in some situations

ref https://linear.app/tryghost/issue/ONC-197

- YouTube has started responding to video page requests with localised content when requested from certain IPs, with that localised content not containing the required `<link rel="alternate" ...>` tag pointing to the oembed endpoint
- we were fetching video pages rather than the oembed endpoint for YouTube Live URLs because they are not recognised by the oembed extraction library we use
- by modifying the URL from a live URL to a watch URL before we perform oembed lookup/extraction we are able to bypass the (localised) page fetch and instead grab the oembed content directly

ghost/oembed-service/lib/OEmbedService.js

@@ -373,6 +373,18 @@ class OEmbedService {
         try {
             const urlObject = new URL(url);
 
+            // YouTube has started not returning oembed <link>tags for some live URLs
+            // when fetched from an IP address that's in a non-EN region.
+            // We convert live URLs to watch URLs so we can go straight to the
+            // oembed request via a known provider rather than going through the page fetch routine.
+            const ytLiveRegex = /^\/live\/([a-zA-Z0-9_-]+)$/;
+            if (urlObject.hostname === 'www.youtube.com' && ytLiveRegex.test(urlObject.pathname)) {
+                const videoId = ytLiveRegex.exec(urlObject.pathname)[1];
+                urlObject.pathname = '/watch';
+                urlObject.searchParams.set('v', videoId);
+                url = urlObject.toString();
+            }
+
             // Trimming solves the difference of url validation between `new URL(url)`
             // and metascraper.
             url = url.trim();

ghost/oembed-service/test/oembed-service.test.js

@@ -172,5 +172,31 @@ describe('oembed-service', function () {
             assert.equal(response.url, 'https://www.example.com');
             assert.equal(response.metadata.title, 'Example');
         });
+
+        it('converts YT live URLs to watch URLs', async function () {
+            nock('https://www.youtube.com')
+                .get('/oembed')
+                .query((query) => {
+                    // Ensure the URL is converted to a watch URL and retains existing query params.
+                    const actual = query.url;
+                    const expected = 'https://youtube.com/watch?param=existing&v=1234';
+
+                    assert.equal(actual, expected, 'URL passed to oembed endpoint is incorrect');
+
+                    return actual === expected;
+                })
+                .reply(200, {
+                    type: 'rich',
+                    version: '1.0',
+                    title: 'Test Title',
+                    author_name: 'Test Author',
+                    author_url: 'https://example.com/user/testauthor',
+                    html: '<iframe src="https://www.example.com/embed"></iframe>',
+                    width: 640,
+                    height: null
+                });
+
+            await oembedService.fetchOembedDataFromUrl('https://www.youtube.com/live/1234?param=existing');
+        });
     });
 });