Convert DTOs to immutable record types

[Copilot]

All DTOs were mutable classes. Converted to immutable records for value semantics, compile-time safety, and reduced boilerplate.

Changes

Response DTOs → Positional records:

// Before
public class ItemResponseDto
{
    public Guid Id { get; set; }
    public required string Sku { get; set; }
    public required string Description { get; set; }
    public bool IsActive { get; set; }
}

// After
public record ItemResponseDto(Guid Id, string Sku, string Description, bool IsActive);

Post DTOs → Nominal records with init:

// Before
public class PostItemDto
{
    public required string Sku { get; set; }
    // ...
}

// After
public record PostItemDto
{
    public required string Sku { get; init; }
    // ...
}

Maintained mutable set on service-populated fields (Id, DateCreated, CreatedBy) to preserve existing service layer patterns.

DTOs converted

• CustomerResponseDto, PostCustomerDto
• VendorResponseDto, PostVendorDto
• ItemResponseDto, PostItemDto
• AuthenticationResponse (already record)

Updated explicit conversion operators, example files, and tests to use positional syntax and immutable initialization patterns.

Original prompt

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This section details on the original issue you should resolve

<issue_title>DTOs should be immutable record types instead of classes.</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes DTOs should be immutable record types instead of classes. #35

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