NullableAttributeEdmPropertyConvention incorrectly marks Nullable<T> value type properties as non-nullable

Description

ODataConventionModelBuilder incorrectly marks Nullable<T> value type properties (e.g. DateOnly?, int?, TimeOnly?) as Nullable="false" in the EDM model. This is a regression introduced by the NRT (Non-nullable Reference Types) convention support added in #16 / PR #17.

Root Cause

The NullableAttributeEdmPropertyConvention misinterprets the C# compiler's [Nullable] attribute for value types:

1. Initial default is correct — StructuralPropertyConfiguration constructor calls EdmLibHelpers.IsNullable(), which correctly returns true for Nullable<T> value types.

2. Convention overrides it — NullableAttributeEdmPropertyConvention.Apply() reads the [Nullable] attribute emitted by the C# compiler. For Nullable<DateOnly> in a #nullable enable context, the compiler emits NullableAttribute with flag 1 (non-nullable), because value type nullability is expressed through Nullable<T> in the CLR type system, not through NRT annotations. The convention interprets flag 1 as "make it non-nullable," overriding the correct default.

Reference types (e.g. string?) are not affected because their [Nullable] flags correctly indicate nullability (2 = nullable).

Reproduce

#nullable enable

public sealed record PersonResponse
{
    public required Guid Id { get; init; }
    public required string FirstName { get; init; }
    public DateOnly? DateOfBirth { get; init; }  // <-- incorrectly becomes Nullable="false"
    public string? Email { get; init; }           // <-- correctly remains nullable
}

var modelBuilder = new ODataConventionModelBuilder();
modelBuilder.EntitySet<PersonResponse>("persons");
var model = modelBuilder.GetEdmModel();

Expected metadata

<Property Name="DateOfBirth" Type="Edm.Date" />
<!-- or explicitly: Nullable="true" -->

Actual metadata

<Property Name="DateOfBirth" Type="Edm.Date" Nullable="false" />

Impact

When ASP.NET Core OData registers input formatters, the non-nullable EDM property causes the OData deserializer to reject null or missing values for DateOnly? properties in POST/PUT/PATCH requests, even though the CLR type explicitly allows null.

Workaround

Use OnModelCreating to globally fix all Nullable<T> value type properties after conventions run:

var modelBuilder = new ODataConventionModelBuilder
{
    OnModelCreating = builder =>
    {
        foreach (var type in builder.StructuralTypes)
        {
            foreach (var property in type.Properties)
            {
                if (property is PrimitivePropertyConfiguration primitive
                    && Nullable.GetUnderlyingType(property.PropertyInfo.PropertyType) is not null)
                {
                    primitive.IsNullable();
                }
            }
        }
    },
};

Suggested Fix

In NullableAttributeEdmPropertyConvention.Apply(), skip properties whose PropertyInfo.PropertyType is a Nullable<T> value type, since their nullability is already correctly determined by EdmLibHelpers.IsNullable() from the CLR type system.

Environment

• Microsoft.OData.ModelBuilder 2.0.0
• Microsoft.AspNetCore.OData 9.4.1
• .NET 10 (also reproducible on .NET 8/9)
• #nullable enable context

Start at NullableAttributeEdmPropertyConvention.Apply() and inspect how it handles PropertyInfo.PropertyType for Nullable value types. Reproduce the DateOnly? case with ODataConventionModelBuilder, then verify the generated EDM keeps the property nullable and that nullable reference type handling remains unchanged.