test: cover 400 validation branches in request API

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/app/api/requests/route.test.ts

@@ -25,6 +25,26 @@ describe("requests API", () => {
     expect(res.status).toBe(400);
   });
 
+  it("rejects a malformed JSON body", async () => {
+    const req = new Request("http://localhost/api/requests", {
+      method: "POST",
+      headers: { "content-type": "application/json" },
+      body: "{not json",
+    });
+    const res = await POST(req);
+    expect(res.status).toBe(400);
+  });
+
+  it("rejects an empty/whitespace-only artist", async () => {
+    const res = await POST(postReq({ artist: "   ", album: "X" }));
+    expect(res.status).toBe(400);
+  });
+
+  it("rejects a non-string artist field", async () => {
+    const res = await POST(postReq({ artist: 123, album: "X" }));
+    expect(res.status).toBe(400);
+  });
+
   it("lists requests newest first with job state", async () => {
     await POST(postReq({ artist: "A", album: "1" }));
     await POST(postReq({ artist: "B", album: "2" }));