Fix netstandard2.0 build: use Split(char) instead of Split(char, StringSplitOptions)

Agent-Logs-Url: https://github.com/modelcontextprotocol/csharp-sdk/sessions/5e730596-6a29-45ba-8610-4bc03424893e

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

Author: Copilot

src/ModelContextProtocol.Core/Authentication/ClientOAuthProvider.cs

@@ -736,10 +736,20 @@ private async Task PerformDynamicClientRegistrationAsync(
         // all previously requested scopes to prevent losing previously granted permissions.
         if (_lastRequestedScopes is not null && newScopes is not null)
         {
-            var combined = new HashSet<string>(_lastRequestedScopes.Split(' ', StringSplitOptions.RemoveEmptyEntries), StringComparer.Ordinal);
-            foreach (var scope in newScopes.Split(' ', StringSplitOptions.RemoveEmptyEntries))
+            var combined = new HashSet<string>(StringComparer.Ordinal);
+            foreach (var scope in _lastRequestedScopes.Split(' '))
             {
-                combined.Add(scope);
+                if (scope.Length > 0)
+                {
+                    combined.Add(scope);
+                }
+            }
+            foreach (var scope in newScopes.Split(' '))
+            {
+                if (scope.Length > 0)
+                {
+                    combined.Add(scope);
+                }
             }
             newScopes = string.Join(" ", combined);
         }