fix(tests): stabilize diffs localReq headers (supersedes #39063)

Co-authored-by: Shennng <Shennng@users.noreply.github.com>

extensions/diffs/index.test.ts

@@ -140,9 +140,14 @@ describe("diffs plugin registration", () => {
   });
 });
 
-function localReq(input: { method: string; url: string }): IncomingMessage {
+function localReq(input: {
+  method: string;
+  url: string;
+  headers?: IncomingMessage["headers"];
+}): IncomingMessage {
   return {
     ...input,
+    headers: input.headers ?? {},
     socket: { remoteAddress: "127.0.0.1" },
   } as unknown as IncomingMessage;
 }