Replace header-based staleTime tests with behavior tests
- Delete old tests that only verified x-nextjs-stale-time header
- Add new test suite that tests actual cache behavior (refetch vs no-refetch)
- Test cases: override global staleTimes, nested layouts (inner wins), layout inheritance
- Remove staleTime=0 test since minimum enforced is 30 seconds (RUNTIME_PREFETCH_DYNAMIC_STALE)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
