Add X-Accel-Buffering header to SSE responses (#1294)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

src/ModelContextProtocol.AspNetCore/StreamableHttpHandler.cs

@@ -339,6 +339,7 @@ internal sealed class StreamableHttpHandler(
 
         // Make sure we disable all response buffering for SSE.
         context.Response.Headers.ContentEncoding = "identity";
+        context.Response.Headers["X-Accel-Buffering"] = "no";
         context.Features.GetRequiredFeature<IHttpResponseBodyFeature>().DisableBuffering();
     }