Use Uri.Compare for AS issuer comparison (robustness improvement)

Agent-Logs-Url: https://github.com/modelcontextprotocol/csharp-sdk/sessions/101fdc47-ecb5-4163-829b-0b320592a4be

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.Core/Authentication/ClientOAuthProvider.cs

@@ -1032,7 +1032,10 @@ private void HandleAuthorizationServerChange(Uri currentIssuer)
     }
 
     private static bool UrisAreEquivalent(Uri a, Uri b) =>
-        string.Equals(a.AbsoluteUri.TrimEnd('/'), b.AbsoluteUri.TrimEnd('/'), StringComparison.OrdinalIgnoreCase);
+        Uri.Compare(a, b,
+            UriComponents.Scheme | UriComponents.Host | UriComponents.Port | UriComponents.Path,
+            UriFormat.SafeUnescaped,
+            StringComparison.OrdinalIgnoreCase) == 0;
 
     [DoesNotReturn]
     private static void ThrowFailedToHandleUnauthorizedResponse(string message) =>