mcp: use http.ResponseController to ensure writes are flushed (#870)

If the ResponseWriter is a wrapped one (which is quite common when using
middleware to log things like response sizes), the direct cast to
`http.Flusher` doesn't find the flush capability. Instead, use
`http.ResponseController` as introduced in go1.20 which handles
unwrapping to find the base response writer.

mcp/streamable.go

@@ -1053,9 +1053,9 @@ func (c *streamableServerConn) acquireStream(ctx context.Context, w http.Respons
 		// Issue #410: the standalone SSE stream is likely not to receive messages
 		// for a long time. Ensure that headers are flushed.
 		w.WriteHeader(http.StatusOK)
-		if f, ok := w.(http.Flusher); ok {
-			f.Flush()
-		}
+		rc := http.NewResponseController(w)
+		// Ignore returned error as flushing is best-effort.
+		_ = rc.Flush()
 	}
 
 	for _, data := range toReplay {