Fix compilation errors in new docs

Author: jeffhandley

docs/concepts/ping/ping.md

@@ -16,7 +16,7 @@ MCP includes a [ping mechanism] that allows either side of a connection to verif
 Use the <xref:ModelContextProtocol.Client.McpClient.PingAsync*> method to verify the server is responsive:
 
 ```csharp
-await client.PingAsync(cancellationToken);
+await client.PingAsync(cancellationToken: cancellationToken);
 ```
 
 ### Automatic ping handling

docs/concepts/resources/resources.md

@@ -215,7 +215,7 @@ builder.Services.AddMcpServer()
         if (ctx.Params?.Uri is { } uri)
         {
             // Track the subscription (e.g., in a concurrent dictionary)
-            subscriptions[context.Server.SessionId].TryAdd(uri, 0);
+            subscriptions[ctx.Server.SessionId].TryAdd(uri, 0);
         }
         return new EmptyResult();
     })

docs/concepts/sampling/sampling.md

@@ -100,11 +100,8 @@ McpClientOptions options = new()
         SamplingHandler = async (request, progress, cancellationToken) =>
         {
             // Forward to your LLM, apply content filtering, etc.
-            string prompt = request?.Messages?.LastOrDefault()?.Content switch
-            {
-                TextContentBlock text => text.Text,
-                _ => string.Empty
-            };
+            string prompt = request?.Messages?.LastOrDefault()?.Content
+                .OfType<TextContentBlock>().FirstOrDefault()?.Text ?? string.Empty;
 
             return new CreateMessageResult
             {