Request deserialization errors kill the connection instead of returning JSON-RPC error

[TrevorBurnham]

Summary

When a JSON-RPC request matches a registered method but fails to deserialize its params (e.g., a missing or misspelled field), the error propagates through the handler chain and kills the entire connection. The client receives no response and hangs indefinitely.

The expected behavior is to send a JSON-RPC -32602 Invalid params error response for that request and continue serving the connection.

Reproduction

Using sacp 10.1.0, register a handler for a request type (e.g., PromptRequest which expects a prompt field). Send a request with the correct method but a wrong field name:

{
  "jsonrpc": "2.0",
  "id": 3,
  "method": "session/prompt",
  "params": {
    "sessionId": "sess_abc123",
    "content": [{"type": "text", "text": "hello"}]
  }
}

The server logs a parse error and the connection dies. The client never receives a response.

Root cause

In jsonrpc/handlers.rs, RequestHandler::handle_message returns Err(err) when Req::parse_message returns Some(Err(..)):

Some(Err(err)) => {
    tracing::trace!(?err, "RequestHandler::handle_request: parse errored");
    Err(err)
}

This error propagates via ? through:

1. dispatch_request in incoming_actor.rs
2. The main loop of incoming_protocol_actor
3. Up to serve(), which returns the error

The connection is then torn down.

Suggested fix

When a request has an ID and its params fail to deserialize, send a JSON-RPC error response (e.g., -32602 Invalid params) to the client for that specific request, rather than propagating the error and killing the connection.