feat(features): contract suites for all repository interfaces

Each repository interface now has a contract suite under
src/__contracts__/. Both Mock and Payload implementations run the
same suite, eliminating mock-vs-real drift. Payload impls back the
contract with an in-memory stub via vi.mock('payload') + a small
buildPayloadStub helper.

Spec: §5.2, §6.4

packages/auth/src/infrastructure/repositories/mock-users.repository.test.ts

@@ -0,0 +1,8 @@
+import { describe } from "vitest";
+import { MockUsersRepository } from "@/infrastructure/repositories/mock-users.repository";
+import { usersRepositoryContract } from "@/__contracts__/users-repository.contract";
+
+describe("MockUsersRepository", () => {
+  // Start with empty store so contract tests run from a clean slate.
+  usersRepositoryContract.run(() => new MockUsersRepository([]));
+});

packages/auth/src/infrastructure/repositories/mock-users.repository.ts

@@ -4,12 +4,18 @@ import { injectable } from "inversify";
 import type { IUsersRepository } from "../../application/repositories/users-repository.interface";
 import type { User } from "../../entities/user";
 
+const DEFAULT_SEED: User[] = [
+  { id: "1", username: "alice", passwordHash: "hashed_password_alice" },
+  { id: "2", username: "bob", passwordHash: "hashed_password_bob" },
+];
+
 @injectable()
 export class MockUsersRepository implements IUsersRepository {
-  private _users: User[] = [
-    { id: "1", username: "alice", passwordHash: "hashed_password_alice" },
-    { id: "2", username: "bob", passwordHash: "hashed_password_bob" },
-  ];
+  private _users: User[];
+
+  constructor(initialUsers: User[] = DEFAULT_SEED) {
+    this._users = [...initialUsers];
+  }
 
   async getUser(id: string): Promise<User | undefined> {
     return this._users.find((u) => u.id === id);