Honor validation attributes for null endpoint parameters

[Copilot]

Honor validation attributes for null endpoint parameters

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Run validation attributes for null-bound endpoint parameters

Description

Minimal API endpoint validation skipped parameter validation entirely when model binding produced null. This caused custom validation attributes on nullable value-type parameters to be ignored for omitted query values.

• Behavior change

  • Remove the endpoint-filter early exit for null arguments so parameter validation still runs.
  • Preserve ValidationContext creation for null-bound parameters by supplying a stable placeholder instance.

• Coverage

  • Add a routing unit test for a nullable value-type endpoint parameter with a custom ValidationAttribute that always fails when the query value is omitted.

• Example

  app.MapGet("/get", ([FromQuery, MyVal] int? id) => "ok");
  
  internal sealed class MyValAttribute : ValidationAttribute
  {
      protected override ValidationResult IsValid(object value, ValidationContext validationContext)
          => new("Always failing attribute.");
  }

  Before this change, GET /get bypassed validation and reached the handler. With this change, it produces the expected validation problem response.