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/navigation/src/__contracts__/header-repository.contract.ts

@@ -1,12 +1,26 @@
 import { it, expect, beforeEach } from "vitest";
 import { defineContractSuite } from "@repo/core-testing/contract";
 import type { IHeaderRepository } from "../application/repositories/header-repository.interface.js";
+import type { Header } from "../entities/header.js";
+
+/**
+ * Known fixtures that every implementation's `buildSubject` must pre-seed.
+ * Exported so that test files can pass them to `MockHeaderRepository` or the
+ * Payload stub without duplicating definitions.
+ */
+export const CONTRACT_HEADER_SEED: Header = {
+  items: [
+    { label: "Home", href: "/", external: false },
+    { label: "Blog", href: "/blog", external: false },
+    { label: "Docs", href: "/docs", external: true },
+  ],
+};
 
 /**
  * Contract for IHeaderRepository.
  *
  * Header is a singleton (Payload Global). The interface exposes only
- * getHeader(). The contract verifies the shape of the return value.
+ * getHeader(). The contract verifies the shape, count, and order of items.
  */
 export const headerRepositoryContract =
   defineContractSuite<IHeaderRepository>(
@@ -20,10 +34,24 @@ export const headerRepositoryContract =
 
       // --- getHeader ---
 
-      it("getHeader returns an object with an items array", async () => {
+      it("getHeader returns an object with an items array of the seeded length", async () => {
         const header = await repo.getHeader();
         expect(header).toBeDefined();
         expect(header.items).toBeInstanceOf(Array);
+        expect(header.items).toHaveLength(3);
+      });
+
+      it("getHeader items appear in the seeded order with correct shape", async () => {
+        const header = await repo.getHeader();
+        expect(header.items[0]?.label).toBe("Home");
+        expect(header.items[0]?.href).toBe("/");
+        expect(header.items[0]?.external).toBe(false);
+        expect(header.items[1]?.label).toBe("Blog");
+        expect(header.items[1]?.href).toBe("/blog");
+        expect(header.items[1]?.external).toBe(false);
+        expect(header.items[2]?.label).toBe("Docs");
+        expect(header.items[2]?.href).toBe("/docs");
+        expect(header.items[2]?.external).toBe(true);
       });
 
       it("getHeader items have label, href, and external fields", async () => {