Add shared test helper utilities (tests/helpers)

[Armolas]

Consolidates duplicated mock-response/request builders, JWT test-token generation, mock TypeORM repository wiring, and success/error response assertions into tests/helpers, and refactors middleware.test.ts and WalletController.test.ts to use them as a proof of concept.

Description

Adds a shared tests/helpers/ module (auth.ts, db.ts, http.ts, assertions.ts, index.ts) so tests no longer duplicate the same boilerplate. setupTestDb()/teardownTestDb() wrap the existing jest.mock('AppDataSource') convention (no real Postgres test DB exists in this repo, and CI never spins one up) instead of standing up real infra, and createAuthenticatedRequest() is a mock request builder (no supertest) matching how every existing controller/middleware test already works. middleware.test.ts and WalletController.test.ts are refactored to use the new helpers as a proof of concept, and src/__tests__/testHelpers.test.ts covers every exported helper directly, including a round-trip through the real authMiddleware.requireAuth.

Fixes #120

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation update

Checklist:

• My code follows the coding conventions
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

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