fix(tests): address Task 4 code review feedback

- Deprecate mockPayloadModule with throw guard (hoisting incompatible)
- Replace `as never` with stubPayloadConfig in payload-articles test (consistency)
- Tighten pages contract to use toHaveLength (exact assertions)
- Header contract: define CONTRACT_HEADER_SEED, assert item count + order

Reviewer: superpowers:code-reviewer (Task 4 of Plan 7).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/core-testing/src/payload/mock-payload-module.ts

@@ -1,10 +1,27 @@
-import { vi } from "vitest";
 import type { Payload } from "payload";
 
-// Helper to mock the `payload` package with a custom getPayload impl.
-// Call inside a test file BEFORE importing the SUT.
+/**
+ * @deprecated DO NOT USE — hoisting incompatible.
+ *
+ * Vitest statically hoists `vi.mock()` calls to the top of the file.
+ * This helper wraps `vi.mock()` inside a regular function body, so the
+ * hoist never fires — the mock is installed too late and the real
+ * `payload` module loads before being intercepted.
+ *
+ * Instead, write at the TOP LEVEL of your test file:
+ *
+ *     vi.mock("payload", () => ({ getPayload: vi.fn() }));
+ *
+ *     // then in your test:
+ *     const { getPayload } = await import("payload");
+ *     (getPayload as ReturnType<typeof vi.fn>).mockResolvedValue(yourStub);
+ *
+ * This export is preserved only so existing imports don't break and
+ * agents searching for it find this warning.
+ */
 export function mockPayloadModule(impl: Partial<Payload>): void {
-  vi.mock("payload", () => ({
-    getPayload: vi.fn().mockResolvedValue(impl),
-  }));
+  void impl;
+  throw new Error(
+    "mockPayloadModule is deprecated and broken; see JSDoc. Call vi.mock('payload', ...) at the top of your test file instead.",
+  );
 }