Enable SendTo/Multicast AccessDenied tests on Apple platforms

[Copilot]

SendTo<T>.Datagram_UDP_AccessDenied_Throws_DoesNotBind and SocketOptionNameTest.MulticastInterface_Set_AnyInterface_Succeeds were neutered by a runtime SkipTestException workaround: on Apple platforms they skipped instead of asserting whenever a UDP send to the broadcast address returned HostUnreachable (the macOS 15+ "Local Network" privacy permission surfaces EHOSTUNREACH rather than EACCES). A skip is not a pass, so the coverage was effectively disabled.

Changes

• SendTo.cs — Removed the HostUnreachable/SkipTestException branch so the test asserts AccessDenied again; reverted [ConditionalFact] back to [Fact] (the conditional existed only to honor the skip).
• SocketOptionNameTest.cs — Removed the equivalent try/catch skip from MulticastInterface_Set_AnyInterface_Succeeds, restoring the plain await MulticastInterface_Set_Helper(0); body.

[Fact]
[SkipOnPlatform(TestPlatforms.FreeBSD, "FreeBSD allows sendto() to broadcast")]
public async Task Datagram_UDP_AccessDenied_Throws_DoesNotBind()
{
    IPEndPoint invalidEndpoint = new IPEndPoint(IPAddress.Broadcast, 1234);
    using var socket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
    byte[] buffer = new byte[32];

    SocketException e = await Assert.ThrowsAnyAsync<SocketException>(() => SendToAsync(socket, new ArraySegment<byte>(buffer), invalidEndpoint));
    Assert.Equal(SocketError.AccessDenied, e.SocketErrorCode);
    Assert.Null(socket.LocalEndPoint);
}

Notes for reviewers

• These are test-only changes. The purpose is to let CI report the actual Apple-leg outcome on the new macOS queues.
• The correct long-term fix is infrastructure: granting the Helix macOS runners the "Local Network" permission. If the Apple legs still surface HostUnreachable, that permission is not yet in place and enablement should wait on it.
• Verified locally on Linux: SendTo_SyncSpan.Datagram_UDP_AccessDenied_Throws_DoesNotBind passes. MulticastInterface_Set_AnyInterface_Succeeds could not be exercised in the sandbox (container blocks broadcast/multicast sends, failing in CreateBoundUdpSocket before the edited code) — an environment limitation, not a regression.

• Fixes [macOS] SendTo_.*.Datagram_UDP_AccessDenied_Throws_DoesNotBind cannot reach host #114450

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[kotlarmilos]

/azp run runtime-ioslike

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