Spec Incompatibility on

It seems that a body is being sent in response to:

```
{
  "jsonrpc": "2.0",
  "method": "notifications/initialized"
}
```

The response body is:
```
{"jsonrpc":"2.0","method":"notifications/initialized"}
```

The spec indicates that there should not be a body, only a `202 Accepted` with an empty body.

See: https://github.com/modelcontextprotocol/rust-sdk/issues/455