Handle 202 Accepted response in HTTP client

Per the Streamable HTTP spec, a server returns 202 Accepted with no
body as the ACK for a JSON-RPC notification or response. The client
previously errored on 202 because it fell through Content-Type
dispatch. Short-circuit to return nil before Content-Type handling so
fire-and-forget messages (e.g. notifications/initialized) no longer
raise.

https://modelcontextprotocol.io/specification/2025-11-25/basic/transports#sending-messages-to-the-server

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

examples/streamable_http_server.rb

@@ -171,7 +171,7 @@ puts <<~MESSAGE
      curl -i http://localhost:9393 -H "Mcp-Session-Id: YOUR_SESSION_ID" \\
        --json '{"jsonrpc":"2.0","method":"tools/call","id":3,"params":{"name":"notification_tool","arguments":{"message":"Hello SSE!", "delay": 2}}}'
 
-  Note: When an SSE stream is active, tool responses will appear in the SSE stream and the POST request will return {"accepted": true}
+  Note: When an SSE stream is active, tool responses will appear in the SSE stream and the POST request will return 202 Accepted with no body.
 
   Press Ctrl+C to stop the server
 MESSAGE