Fix support for sending server notifications with request-specific McpServer instances used outside their original scope. (#832)

* Fix support for sending server notifications with request-specific McpServer instances used outside their original scope.

* Address feedback.

* Update src/ModelContextProtocol.Core/Server/SseWriter.cs

src/ModelContextProtocol.Core/Server/StreamableHttpServerTransport.cs

@@ -131,6 +131,7 @@ public sealed class StreamableHttpServerTransport : ITransport
             throw new InvalidOperationException("Unsolicited server to client messages are not supported in stateless mode.");
         }
 
+        // If the underlying writer has been disposed, just drop the message.
         await _sseWriter.SendMessageAsync(message, cancellationToken).ConfigureAwait(false);
     }