fix(validation): compare date component to reject same-day birth dates (#344)

- Change DateOfBirth predicate from `date < DateTime.UtcNow` to
  `date!.Value.Date < DateTime.UtcNow.Date` to reject same-day dates
- Access nullable value via `.Value.Date` consistently in both Must
  lambdas, safe within the When(HasValue) guard

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: nanotaboada

src/Dotnet.Samples.AspNetCore.WebApi/Validators/PlayerRequestModelValidator.cs

@@ -45,9 +45,11 @@ public PlayerRequestModelValidator(IPlayerRepository playerRepository)
             () =>
             {
                 RuleFor(player => player.DateOfBirth)
-                    .Must(date => date < DateTime.UtcNow)
+                    .Must(date => date!.Value.Date < DateTime.UtcNow.Date)
                     .WithMessage("DateOfBirth must be a date in the past.")
-                    .Must(date => date >= new DateTime(1900, 1, 1, 0, 0, 0, DateTimeKind.Utc))
+                    .Must(date =>
+                        date!.Value.Date >= new DateTime(1900, 1, 1, 0, 0, 0, DateTimeKind.Utc)
+                    )
                     .WithMessage("DateOfBirth must be on or after January 1, 1900.");
             }
         );