Fix handling response when sending notifications in examples/streamable_http_client.rb

Even though the 'tools/call' request to 'notification_tool' is successful and an 'MCP::Tool::Response' is returned, the logger still prints 'Error:'.

```
% ruby examples/streamable_http_client.rb
=== MCP SSE Test Client ===

[CLIENT] INFO 15:41:04.696 - Initializing session...
[CLIENT] INFO 15:41:04.698 - Session initialized: SESSION_ID
[CLIENT] INFO 15:41:04.698 - Server info: {"name" => "sse_test_server", "version" => "0.1.0"}
[CLIENT] INFO 15:41:04.698 - Connecting to SSE stream...
[CLIENT] INFO 15:41:04.698 - SSE stream connected successfully

=== Available Actions ===
1. Send custom notification
2. Test echo
3. List tools
0. Exit

Choose an action: 1
Enter notification message: notification test
Enter delay in seconds (0 for immediate):
[CLIENT] INFO 15:41:11.248 - SSE data: {"jsonrpc":"2.0","id":"0f657eeb-3e05-47bb-b7b4-898e2cb503af","result":{"content":[{"type":"text","text":"Notification: notification test (timestamp: 2025-06-21T15:41:11+09:00)"}],"isError":false}}
[CLIENT] ERROR 15:41:11.249 - Error:

=== Available Actions ===
1. Send custom notification
2. Test echo
3. List tools
0. Exit
```

It is probably trying to output an MCP::Tool::Response, but these appear in the SSE stream. Therefore, the response it is handling this time should either be accepted or an error.

I modified it so that it could handle them.

examples/streamable_http_client.rb

@@ -163,9 +163,8 @@ def main
           },
         },
       )
-
-      if response[:body]["result"]
-        logger.info("Notification tool response: #{response[:body]["result"]["content"]}")
+      if response[:body]["accepted"]
+        logger.info("Notification sent successfully")
       else
         logger.error("Error: #{response[:body]["error"]}")
       end