test(atomic-a11y): add comprehensive unit test suite for reporter

[y-lakhdar]

Summary

Adds 247 unit tests across 12 test files covering the full atomic-a11y reporter pipeline. Split out from #7123 to keep the reporter source PR reviewable.

What's Included

• 14 new files: 12 test files + shared mock factories + vitest config
• 3,367 lines of test code (additions only, no source changes)
• 247 tests across all reporter modules

Test Files

File	Tests	Module
guards.test.ts	30	Type guard utilities
sorting.test.ts	12	Result sorting logic
shard-resolution.test.ts	28	CI shard detection
criterion-metadata.test.ts	5	WCAG criteria lookup
axe-rule-mappings.test.ts	14	axe-core → WCAG mappings
storybook-extraction.test.ts	31	Component/story name parsing
error-parsing.test.ts	29	axe error → structured result
axe-integration.test.ts	34	End-to-end axe result processing
reporter-utils.test.ts	20	File I/O and package metadata
summary.test.ts	18	Report summary generation
report-builder.test.ts	10	Full report assembly
vitest-a11y-reporter.test.ts	16	Vitest reporter lifecycle
helpers/mock-factories.ts	—	Shared mock factory functions
vitest.config.js	—	Test configuration

Testing Conventions

• ESM-only imports (.js extensions)
• .test.ts naming (not .spec.ts)
• Public interface testing only — no as unknown as private access
• Mock factories using {...defaults, ...overrides} pattern
• One behavior per test, descriptive should ... names

How to Run

cd packages/atomic-a11y && pnpm test

Related

• Reporter source: feat(atomic-a11y): add vitest a11y reporter #7123
• E2E smoke test: separate PR on feat/a11y-reporter-e2e-smoke-test