feat: add support for MCP ping

[cmeerw]

Fixes #353

[DeusData]

Thank you, @cmeerw! 🙏 This directly fixes #353 — clients like crush send a ping and fork a second server process if they don't get a success response, so a correct empty-result reply matters. Returning {} is exactly the MCP-spec response for ping, and heap_strdup keeps the ownership consistent with the other result_json branches. Verified locally: build clean, all 3,617 tests pass. Merging via squash — authorship preserved. Closes #353. Thanks for making the MCP surface more spec-compliant!