Add ID Validation

[robinmanuelthiel]

No description provided.

[Copilot]

Pull Request Overview

This PR adds ID validation to the BasketItem model by implementing checks for both the Id and ProductId properties to ensure they are not null, empty, or whitespace.

• Adds validation for the Id property to ensure it's not empty or whitespace
• Adds validation for the ProductId property to ensure it's not empty
• Integrates these validations into the existing Validate method

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[Copilot]

The error message incorrectly references 'Quantity' when validating ProductId. It should be 'ProductId must not be empty' instead.

Suggested change

	results.Add(new ValidationResult("Quantity must not be empty", new[] { "ProductId" }));
	results.Add(new ValidationResult("ProductId must not be empty", new[] { "ProductId" }));

[Copilot]

Using ToString() on ProductId will never return null or whitespace since ToString() always returns a string representation. For GUID validation, check if ProductId equals Guid.Empty instead.

Suggested change

	// New validation for Id
	if (string.IsNullOrWhiteSpace(ProductId.ToString()))
	{
	results.Add(new ValidationResult("Quantity must not be empty", new[] { "ProductId" }));
	// New validation for ProductId
	if (ProductId <= 0)
	{
	results.Add(new ValidationResult("ProductId must be a positive integer", new[] { "ProductId" }));