fix: return MethodNotFound error for unrecognized LSP requests

When receiving an unsupported LSP method, the server now returns a
proper MethodNotFound (-32601) error response instead of silently
dropping the request. This prevents well-behaved clients from hanging
indefinitely waiting for a response.

Author: hellozzm

lib/ruby_lsp/server.rb

@@ -120,6 +120,14 @@ def process_message(message)
         @global_state.synchronize { @cancelled_requests << message[:params][:id] }
       when nil
         process_response(message) if message[:result]
+      else
+        if message[:id]
+          send_message(Error.new(
+            id: message[:id],
+            code: Constant::ErrorCodes::METHOD_NOT_FOUND,
+            message: "Method not found: #{message[:method]}",
+          ))
+        end
       end
     rescue DelegateRequestError
       send_message(Error.new(id: message[:id], code: DelegateRequestError::CODE, message: "DELEGATE_REQUEST"))