Remove cache revalidation execution mode reads (#96670)
## Summary
- derive foreground stale-cache revalidation from Work Unit state
instead of `WorkStore.executionMode`
- propagate that specific policy through `"use cache"` and
`unstable_cache` scopes so nested cache and fetch calls preserve their
outer behavior
- add coverage for foreground ISR revalidation through nested
`unstable_cache` scopes
This is intended to be a behavior-preserving refactor.
