Only accept HTML <link rel="alternate"> on success

(cherry picked from commit 6ea48be84abdab66301a957c27dd5d84886dfb36)

packages/backend/src/core/activitypub/ApRequestService.ts

@@ -205,7 +205,12 @@ export class ApRequestService {
 		//#region リクエスト先がhtmlかつactivity+jsonへのalternate linkタグがあるとき
 		const contentType = res.headers.get('content-type');
 
-		if ((contentType ?? '').split(';')[0].trimEnd().toLowerCase() === 'text/html' && _followAlternate === true) {
+		if (
+			res.status >= 200
+			&& res.status <= 299
+			&& (contentType ?? '').split(';')[0].trimEnd().toLowerCase() === 'text/html'
+			&& _followAlternate === true
+		) {
 			const html = await res.text();
 			const { window, happyDOM } = new Window({
 				settings: {