test: type the fetch mock so the MB test typechecks (tsc --noEmit clean)

web/src/lib/musicbrainz.test.ts

@@ -9,7 +9,7 @@ afterEach(() => vi.unstubAllGlobals());
 
 describe("musicbrainz module", () => {
   it("searchArtists maps hits and sends a User-Agent", async () => {
-    const fetchMock = vi.fn(() =>
+    const fetchMock = vi.fn((_url: string, _init?: RequestInit) =>
       jsonResponse({ artists: [{ id: "a1", name: "John Mayer", disambiguation: "US singer" }] }),
     );
     vi.stubGlobal("fetch", fetchMock);