Add IsClosedTypeAttribute

[Copilot]

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.

[Copilot]

Pull request overview

This PR adds a new compiler-reserved attribute, System.Runtime.CompilerServices.IsClosedTypeAttribute, to the BCL surface area so the C# compiler can emit it for closed class hierarchies, with corresponding reference assembly and a basic construction test.

Changes:

• Added IsClosedTypeAttribute implementation in System.Private.CoreLib with [EditorBrowsable(Never)] and class-only AttributeUsage.
• Updated the System.Runtime ref assembly to expose the new attribute publicly.
• Added a small xUnit smoke test to ensure the attribute is constructible.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/Runtime/CompilerServices/AttributesTests.cs	Adds a constructor smoke test for IsClosedTypeAttribute.
src/libraries/System.Runtime/ref/System.Runtime.cs	Adds the public reference declaration for IsClosedTypeAttribute.
src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/IsClosedTypeAttribute.cs	Implements the new compiler-reserved attribute in CoreLib.
src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems	Includes the new source file in the CoreLib shared build items.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-runtime
See info in area-owners.md if you want to be subscribed.