test(lib): cover useApi states and cancellation

[ZaaK0913-coder]

Summary

• Add a focused React hook test suite for useApi.
• Cover loading -> ok, loading -> error, generic fallback errors, null-path skip, path-change refetch, stale response ignoring, and unmount cancellation.
• Keep the change test-only; no production code changes.

Closes #75

Validation

• npm test -- src/lib/__tests__/useApi.test.tsx --runInBand
• npx jest src/lib/__tests__/useApi.test.tsx --coverage --collectCoverageFrom=src/lib/useApi.ts --coverageReporters=text --coverageThreshold "{}" --runInBand (useApi.ts: 100% statements, 100% branches, 100% functions, 100% lines)
• npx eslint src/lib/__tests__/useApi.test.tsx
• npx prettier --check src/lib/__tests__/useApi.test.tsx

Typecheck note

• npm run typecheck was attempted and is currently blocked by pre-existing main-branch errors outside this test-only change:
  • src/app/agents/[agent]/page.tsx: missing Breadcrumb and formatRequests
  • src/app/docs/page.tsx: missing safeHref