Make TestKeyRing.CreateECDsa resilient to transient macOS OSStatus -50 failures

Backport of #125909 to release/8.0-staging.

The automated backport failed because TestKeyRing.cs (introduced by a
later refactor) does not exist on this branch; the CreateECDsa helper
still lives in CoseTestHelpers.cs here. This applies the same fix
(retry ECDsa.Create up to 3 times on macOS when it transiently fails
with OSStatus -50 / CryptographicException HResult -50) to that
location instead.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: ViveliDuCh

src/libraries/System.Security.Cryptography.Cose/tests/CoseTestHelpers.cs

@@ -6,6 +6,7 @@
 using System.Formats.Cbor;
 using System.IO;
 using System.Linq;
+using System.Runtime.InteropServices;
 using Microsoft.IdentityModel.Tokens;
 using Test.Cryptography;
 using Xunit;
@@ -391,7 +392,14 @@ private static ECDsa CreateECDsa(ECParameters parameters, bool includePrivateKey
                 parametersLocalCopy.D = null;
             }
 
-            return ECDsa.Create(parametersLocalCopy);
+            // The Apple Security framework intermittently fails with OSStatus error -50 when creating
+            // EC keys. Retry a few times to make the test robust against this transient failure.
+            ECDsa? result = null;
+            RetryHelper.Execute(
+                () => result = ECDsa.Create(parametersLocalCopy),
+                maxAttempts: 3,
+                retryWhen: e => RuntimeInformation.IsOSPlatform(OSPlatform.OSX) && e is CryptographicException { HResult: -50 });
+            return result!;
         }
 
         private static RSA CreateRSA(bool includePrivateKey)