Fix: address validation fires unconditionally when updating name/email on Admin User page

[Copilot]

Submitting the Admin User personal info form with only name/email changes fails with {"InvalidUserId":["Bad Data"],"UserPersonalUpdateModel.Address":["The Address field is required."]} because address fields carry [Required] regardless of intent.

Changes

UserPersonalUpdateModel

• Remove [Required] from Address, City, Country, PostalCode
• Implement IValidatableObject: address group validation fires only when at least one address field is non-empty; if any is set, all of Address/City/Country/PostalCode must be provided
• Add HasAnyAddressField() helper (reused in handler)

public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
{
    if (!HasAnyAddressField()) yield break;

    if (string.IsNullOrWhiteSpace(Address))
        yield return new ValidationResult("The Address field is required when providing address information.", new[] { nameof(Address) });
    // ... City, Country, PostalCode
}

Admin/User.cshtml.cs — OnPostUpdatePersonalInfoAsync

• Guard UpdateShippingAddress call behind HasAnyAddressField() to avoid null-dereference when no address is submitted

Admin/User.cshtml

• Add asp-validation-summary to the personal info form so server-side validation errors render on-page instead of as a raw JSON BadRequest

Tests

• Unit tests covering: no address fields → no errors; full address → no errors; partial address → errors on missing fields; whitespace-only treated as empty