Enable ESLint on test files

[bntvllnt]

Problem

eslint.config.js excludes tests/** from linting. This creates inconsistent code quality standards between source and test code.

Action

• If intentional (looser test style): document rationale in config comment
• If unintentional: remove exclusion, fix any lint errors in tests
• Ensure CI runs lint on all files

Priority

Low — tests don't ship, but consistency matters.