Fix missing incremental cache in middleware unstable_cache (#89980)
## Summary
- pass `IncrementalCache` into the shared middleware template adapter so
middleware/proxy modules initialize `globalThis.__incrementalCache`
- add regression coverage for `unstable_cache` usage inside both
deprecated `middleware` (edge) and `proxy` (node) fixtures
- verify both e2e suites pass without hitting `Invariant:
incrementalCache missing in unstable_cache`
## Verification
- `pnpm build`
- `NEXT_SKIP_ISOLATE=1 NEXT_TEST_MODE=dev pnpm testonly
test/e2e/app-dir/app-middleware/app-middleware.test.ts`
- `NEXT_SKIP_ISOLATE=1 NEXT_TEST_MODE=dev pnpm testonly
test/e2e/app-dir/app-middleware-proxy/app-middleware-proxy.test.ts`