Remove WorkStore execution mode (#96674)
## Summary
This removes the intermediate `executionMode` from `WorkStore` now that
App Page and App Route select their render or prerender pipeline before
constructing the store.
- Derive ISR error handling from the active `WorkUnitStore`
- Derive fetch configuration processing from the active `WorkUnitStore`,
including treating cache scopes as owners of their cache policy
regardless of the outer render operation
- Use explicit render and prerender WorkStore constructors where
fetch-metric behavior differs
- Keep the App Page render/prerender operation local to the top-level
route-module dispatch
This completes the migration away from global render-mode state on
`WorkStore`, allowing downstream behavior to depend on the more precise
work-unit scope instead.
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