Fix race condition in SSE GET request initialization (#1212)

src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs

@@ -154,11 +154,6 @@ internal sealed class StreamableHttpHandler(
         {
             await using var _ = await session.AcquireReferenceAsync(cancellationToken);
             InitializeSseResponse(context);
-
-            // We should flush headers to indicate a 200 success quickly, because the initialization response
-            // will be sent in response to a different POST request. It might be a while before we send a message
-            // over this response body.
-            await context.Response.Body.FlushAsync(cancellationToken);
             await session.Transport.HandleGetRequestAsync(context.Response.Body, cancellationToken);
         }
         catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)