Enforce input validation with FluentValidation

[nanotaboada]

Description

Improve the API’s request validation by integrating FluentValidation. This will provide a clean and expressive way to define validation rules for input models (e.g., PlayerRequestModel), replacing manual checks in controllers and ensuring a consistent validation strategy.

The FluentValidation.AspNetCore package is no longer being maintained and is now unsupported. We encourage users move away from this package and use the core FluentValidation package with a manual validation approach as detailed at https://docs.fluentvalidation.net/en/latest/aspnet.html

Acceptance Criteria

• Install and configure FluentValidation in the project.
• Create validators for all input models (PlayerRequestModel, etc.).
• Ensure validation rules are complete (e.g., required fields, formats, value ranges).
• Add unit tests to verify validation logic.

Resources

• 🔗 FluentValidation GitHub Repository