fix: ensure error and result are mutually exclusive in JSON-RPC responses (#766)

[randy3k]

Fixes #766

Summary

Per JSON-RPC 2.0 (Section 5):

Either the result member or error member MUST be included, but both members MUST NOT be included.

Previously, Response() unconditionally included result = self in the initial payload list(), so error responses produced both result: null and error: {...}, causing strict clients like Neovim's built-in vim.lsp to terminate the connection.

Changes

• In R/protocol.R (Response()):
  • Construct payload with error when !is.null(self), and with result otherwise.
• In tests/testthat/test-response-json.R:
  • Update expected payloads to reflect mutual exclusivity of result and error.
  • Assert that error responses do not contain result, and success responses (including those with result = NULL) do not contain error.

[renkun-ken]

LGTM Thanks!