Propagate adapter preferred regions (#94123)
### What?
Propagate edge runtime preferredRegion metadata into adapter outputs and
add adapter-config coverage for an edge app route.
### Why?
Adapters need access to preferred region metadata from edge outputs when
generating provider-specific deployment output.
### How?
Copy edge function regions into output.config.preferredRegion and assert
the adapter build-complete output for a fixture route with
preferredRegion set to cdg1.
### Verification
- `pnpm --filter=next build`
- `NEXT_TEST_PREFER_OFFLINE=1 pnpm test-start-webpack
test/production/adapter-config/adapter-config.test.ts`
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