Add unit tests for API login endpoint

[Copilot]

This PR adds unit tests for the login functionality in the API to verify the correct creation of a JWT token.

Implementation details:

• Created a new test project AppVecinos.API.Tests with xUnit, TestHost, and Moq dependencies
• Implemented two test cases for the login endpoint:
  1. Login_WithValidCredentials_ReturnsToken: Verifies that a valid JWT token is returned when correct credentials are provided
  2. Login_WithInvalidCredentials_ReturnsUnauthorized: Verifies that an Unauthorized response is returned when invalid credentials are provided
• Tests use mock services to simulate authentication behavior without requiring a real database
• Added test configuration for JWT settings to enable token generation during tests

Testing strategy:

The tests use the TestHost to create an in-memory test server that simulates HTTP requests to the login endpoint. This approach ensures we're testing the actual endpoint behavior without needing to deploy the API.

The mock setup allows us to test both successful and unsuccessful authentication scenarios by controlling what the NeighborService returns.

Fixes #82.

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