Cover held ordering locks in cfworkers

src/mod.test.ts covered the ordering lock being taken and released, but
never the case where processMessage() finds one already held and returns
shouldProcess: false.  test/mq.test.ts covers that branch by stubbing
get() to return a lock unconditionally; this test takes the lock with a
real processMessage() call and sends a second message for the same key
without releasing it, so it also proves the key that processMessage()
writes is the one it later reads back.

Closes #879

Changelog: none
Assisted-by: Claude Code:claude-opus-5

Author: seongmin36

packages/cfworkers/src/mod.test.ts

@@ -373,4 +373,26 @@ describe("WorkersMessageQueue", () => {
       __fedify_payload__: { id: "test-message" },
     });
   });
+
+  it("processMessage() - returns shouldProcess=false when lock exists", async () => {
+    const orderingKv = new MockKvNamespace();
+    const queue = new WorkersMessageQueue(mockQueue, { orderingKv });
+
+    const first = await queue.processMessage({
+      __fedify_ordering_key__: "key1",
+      __fedify_payload__: { id: "first" },
+    });
+
+    expect(first.shouldProcess).toBe(true);
+    expect(first.message).toEqual({ id: "first" });
+
+    const second = await queue.processMessage({
+      __fedify_ordering_key__: "key1",
+      __fedify_payload__: { id: "second" },
+    });
+
+    expect(second.shouldProcess).toBe(false);
+    expect(second.message).toBeUndefined();
+    expect(second.release).toBeUndefined();
+  });
 });