Preserve false and null response results

[OskarEichler]

Summary

Track whether result was explicitly supplied so JSON-RPC responses preserve false and null instead of replacing both with an empty object. Update the generator, templates, generated Ruby interface, and RBS together.

Reproduction

ResponseMessage.new(id: 1, result: false) and an explicit result: nil currently serialize result as an empty object because truthiness is used to choose the fallback. The focused external model fails on 3.17.0.6 and current main, then passes with this change.

Verification

• Existing suite: 6 runs, 11 assertions, zero failures
• Focused false/null response model passes
• All 326 interface classes and 37 constant modules load and exercise successfully
• Generator rerun is clean
• All Ruby syntax checks pass
• Steep passes
• Rebuilt gem preserves expected generated package contents
• RuboCop LSP initialize/shutdown integration passes
• Rails 8.1.3.1 loads the candidate

No tests were changed. This source-only patch was identified and verified during an AI-assisted dependency audit.