test: enforce 100% code coverage

[Puppo]

Summary

• enforce 100% statement, branch, function, and line coverage for authored runtime code
• add focused tests for empty strings, null schemas, and code-generation fallback paths
• keep generated schema validation code outside the coverage scope through .c8rc.json
• leave production runtime code unchanged

Testing

• npm test
• npm run lint

Coverage reports 100% for statements, branches, functions, and lines.

[mcollina]

lgtm