- add unit tests for API auth, display/error utilities, login bootstrap,
model combo mappings, provider validation branches, and usage analytics
- add COVERAGE_PLAN.md and extend CONTRIBUTING.md with coverage notes and
workflow guidance
- update package.json to adjust test:coverage thresholds and add coverage:report;
include c8 as a devDependency
- introduce test scaffolding and ensure compatibility with existing test runners
- align tests with open-sse changes and improve overall test coverage planning
