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/blog/src/infrastructure/repositories/payload-articles.repository.test.ts

@@ -109,8 +109,6 @@ describe("PayloadArticlesRepository", () => {
   // -------------------------------------------------------------------------
 
   describe("Payload doc → domain mapping", () => {
-    const mockConfig = {} as never;
-
     beforeEach(() => {
       vi.clearAllMocks();
     });
@@ -135,7 +133,7 @@ describe("PayloadArticlesRepository", () => {
         find: findMock,
       });
 
-      const repo = new PayloadArticlesRepository(mockConfig);
+      const repo = new PayloadArticlesRepository(stubPayloadConfig);
       const result = await repo.getArticleBySlug("hello");
 
       expect(findMock).toHaveBeenCalledWith({
@@ -157,7 +155,7 @@ describe("PayloadArticlesRepository", () => {
         find: vi.fn().mockResolvedValue({ docs: [] }),
       });
 
-      const repo = new PayloadArticlesRepository(mockConfig);
+      const repo = new PayloadArticlesRepository(stubPayloadConfig);
       const result = await repo.getArticleBySlug("missing");
       expect(result).toBeUndefined();
     });