Implement validation and error handling

[ryanrichard19]

Add validation to your API endpoints to ensure users provide valid data, and implement proper error handling to return informative error messages to your API consumers.

Tasks to complete:

• Add model validation attributes to your classes, ensuring that user input is validated.
• Implement custom validation logic as needed for more complex validation scenarios.
• Add error handling middleware to your API pipeline to catch exceptions and return informative error messages.
• Test validation and error handling to ensure that invalid input is rejected, and appropriate error messages are returned.