fix(v2): enforce method contracts

[benbrandt]

Summary

• keep _ as the custom-extension convention while allowing unrecognized method strings for compatibility with other unstable ACP drafts
• add method-safe, direction-safe v2 batch descriptors
• validate every built-in direct and batch response before returning it
• keep local void handlers encoded as an empty object while rejecting peer null results

Current protocol method literals continue to use their generated contracts; only method names unrecognized by this SDK use the permissive compatibility path. The existing experimental v2 entrypoint and protocol lane remain unchanged.

Testing

• npm run check (840 tests)