docs(testing): Add comprehensive Testing documentation

**Testing Hub (1):**
- TEST_README.md: Central hub for all testing documentation
  - Quick navigation to all testing guides
  - Overview of testing strategies
  - Links to specific testing approaches

**Testing Strategy Guides (4):**
- CLEAN_ARCHITECTURE_TESTING_STRATEGY.md: Complete testing strategy
  - Unit testing per layer (Domain, Application, Infrastructure, Presentation)
  - Integration testing approaches
  - Architecture tests with NetArchTest
  - End-to-End testing strategies
  - Test organization and structure
  - Mocking strategies (Moq, NSubstitute)
  - Test data builders
  - xUnit, NUnit, MSTest examples
  - Code coverage targets (80%+ recommended)
  - CI/CD integration

- API_ENDPOINT_TESTING_GUIDE.md: How to test all API endpoints
  - Testing with Swagger UI
  - PowerShell testing scripts
  - Postman collection examples
  - curl commands for all endpoints
  - Authentication endpoint testing
  - Protected endpoint testing with JWT
  - Token blacklisting testing
  - Admin monitoring endpoint testing
  - Rate limiting verification
  - Error response testing

- TEST_AUTHENTICATION_GUIDE.md: Step-by-step authentication testing
  - Running the application
  - Getting JWT tokens via CQRS endpoints
  - Testing protected endpoints
  - Testing logout and token blacklisting
  - Testing authorization policies (User, Admin)
  - Testing rate limiting
  - Admin monitoring endpoints
  - Troubleshooting guide
  - Complete testing workflow (5 minutes)

**Key Topics Covered:**
- Unit testing strategies per layer
- Integration testing with WebApplicationFactory
- Architecture tests with NetArchTest
- API endpoint testing approaches
- Authentication and authorization testing
- JWT token testing workflows
- Token blacklisting verification
- Rate limiting testing
- CORS testing
- Error handling testing
- Swagger/OpenAPI testing
- PowerShell testing scripts
- Postman integration
- curl command examples
- Mock objects and test doubles
- Test data builders
- Code coverage reporting
- CI/CD test integration
- Test organization best practices

This provides complete testing documentation with practical examples for all layers.

Author: dariemcarlosdev