Add IsClosedTypeAttribute - #128599

Exposes the attribute approved in #128161 (comment), emitted by the C# compiler to mark types declared as closed (see the closed-hierarchies proposal). Per @bartonjs's feedback, the name was changed from ClosedAttribute to IsClosedTypeAttribute since it is not intended to be written by users.

namespace System.Runtime.CompilerServices
{
    [EditorBrowsable(EditorBrowsableState.Never)]
    [AttributeUsage(AttributeTargets.Class, Inherited = false)]
    public sealed class IsClosedTypeAttribute : Attribute
    {
        public IsClosedTypeAttribute() { }
    }
}

Changes:

• New type: src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/IsClosedTypeAttribute.cs, marked [EditorBrowsable(Never)] to match the convention for compiler-only attributes (e.g. RequiresLocationAttribute).
• Build: include the new file in System.Private.CoreLib.Shared.projitems.
• Ref assembly: add the corresponding declaration in src/libraries/System.Runtime/ref/System.Runtime.cs.
• Tests: add a constructor smoke test in AttributesTests.cs next to the existing RequiresLocationAttribute test.
• Feedback fix: remove explicit AllowMultiple = false from AttributeUsage in both implementation and ref declaration to resolve ApiCompat CP0015 (System.Private.CoreLib ref/lib attribute argument mismatch).

AttributeTargets is intentionally restricted to Class only; broadening to interfaces/enums is deferred until those "closed" forms are designed.