Client never answers a server ping sent during the initialize handshake #1362

Describe the bug

This is about the 2025-11-25 initialize handshake. While the client waits for the initialize response, expect_response in service/client.rs drops any ping from the server:

// Server could send pings before handshake
ServerJsonRpcMessage::Request(ref request)
    if matches!(request.request, ServerRequest::PingRequest(_)) =>
{
    tracing::trace!("Received ping request. Ignored.")
}

Nothing is written back, so the server's ping never gets a response. A server that times out its pings can treat the connection as stale and drop it. A ping sent after notifications/initialized is answered normally.

The server side already does this. #745 made the server answer a client ping that arrives before initialize.

To Reproduce (main at 8d226f4)

Over a tokio::io::duplex transport, with both sides on 2025-11-25, a raw server reads initialize, writes a ping, and then writes the InitializeResult:

<- {"jsonrpc":"2.0","id":0,"method":"initialize","params":{"capabilities":{},"clientInfo":{"name":"rmcp","version":"3.5.1"},"protocolVersion":"2025-11-25"}}
-> {"jsonrpc":"2.0","id":"srv-ping-pre","method":"ping"}
-> {"jsonrpc":"2.0","id":0,"result":{"protocolVersion":"2025-11-25","capabilities":{},"serverInfo":{...}}}

After notifications/initialized arrives, it sends a second ping (srv-ping-post) for comparison. Everything the client writes back:

{"jsonrpc":"2.0","method":"notifications/initialized"}
{"jsonrpc":"2.0","id":"srv-ping-post","result":{}}

There is no response for srv-ping-pre.

Expected behavior

The client writes {"jsonrpc":"2.0","id":"srv-ping-pre","result":{}} during the handshake and then carries on waiting for the InitializeResult.

Additional context

The 2025-11-25 lifecycle lets the server send pings before it gets notifications/initialized, and the ping page says the receiver MUST respond promptly with an empty response.

expect_response is also used while waiting for server/discover. 2026-07-28 removes ping and servers there don't send requests, so I'd leave that path as it is and only answer during initialize.

I can open a PR for this.