Keep canonical WebFinger aliases

When a split-origin acct lookup uses a non-canonical username, include the
preferred username on the web-origin host in aliases.  This keeps the fallback
acct response consistent with actor URI lookups while preserving the queried
alias.

#921 (comment)

Assisted-by: Codex:gpt-5.5

Author: dahlia

packages/fedify/src/federation/webfinger.test.ts

@@ -269,6 +269,8 @@ test("handleWebFinger()", async (t) => {
       ? "someone"
       : username === "bar"
       ? "someone2"
+      : username === "qux"
+      ? "someone2"
       : null;
   };
 
@@ -442,6 +444,47 @@ test("handleWebFinger()", async (t) => {
       ],
     });
 
+    u.searchParams.set("resource", "acct:qux@ap.example.com");
+    context = createContext(u);
+    request = context.request;
+    response = await handleWebFinger(request, {
+      context,
+      host: "example.com",
+      actorDispatcher,
+      actorHandleMapper,
+      onNotFound,
+    });
+    assertEquals(response.status, 200);
+    assertEquals(await response.json(), {
+      subject: "acct:bar@example.com",
+      aliases: [
+        "https://ap.example.com/users/someone2",
+        "acct:qux@ap.example.com",
+        "acct:bar@ap.example.com",
+      ],
+      links: [
+        {
+          href: "https://ap.example.com/users/someone2",
+          rel: "self",
+          type: "application/activity+json",
+        },
+        {
+          href: "https://ap.example.com/@someone2",
+          rel: "http://webfinger.net/rel/profile-page",
+        },
+        {
+          href: "https://ap.example.com/@someone2",
+          rel: "alternate",
+          type: "text/html",
+        },
+        {
+          href: "https://ap.example.com/icon.jpg",
+          rel: "http://webfinger.net/rel/avatar",
+          type: "image/jpeg",
+        },
+      ],
+    });
+
     u = new URL(url);
     u.searchParams.set("resource", "acct:someone@handle.example.com");
     context = createContext(u);

packages/fedify/src/federation/webfinger.ts

@@ -259,6 +259,12 @@ async function handleWebFingerInternal<TContextData>(
   ) {
     subject = `acct:${actor.preferredUsername ?? acctUsername}@${host}`;
     aliases.push(resourceUrl.href);
+    if (
+      actor.preferredUsername != null && actor.preferredUsername !== "" &&
+      actor.preferredUsername !== acctUsername
+    ) {
+      aliases.push(`acct:${actor.preferredUsername}@${context.url.host}`);
+    }
   }
   const jrd: ResourceDescriptor = {
     subject,