Notification initialized parse error

[awgn]

Describe the bug
rmcp::model::JsonRpcMessage fails to deserialize valid JSON-RPC 2.0 notifications that omit the params field. According to the JSON-RPC 2.0 specification, params is optional, but serde throws a "data did not match any variant of untagged enum JsonRpcMessage" error when parsing messages like the standard MCP notifications/initialized notification.

To Reproduce
Steps to reproduce the behavior:

1. Receive (e.g. from inspector) or define a valid JSON-RPC 2.0 notification string without the params key: {"method":"notifications/initialized","jsonrpc":"2.0"}
2. Attempt to deserialize it using serde_json:

   // Attempting to parse the payload
   let payload = b"{\"method\":\"notifications/initialized\",\"jsonrpc\":\"2.0\"}";
   let result: Result<rmcp::model::JsonRpcMessage, _> = serde_json::from_slice(payload);

3. The result is an Err

Additional context
This bug breaks the standard Model Context Protocol (MCP) connection handshake. When the client sends the notifications/initialized message (which takes no parameters) to signal it's ready, the gateway fails to parse it.

Please verify if this is an actual bug in the crate, if I am misusing the library, or if the issue is actually on the inspector's side (in which case, please ignore this report).