Build fails with `[UNRESOLVED_ENTRY] nitro/dist/presets/bun/runtime/bun` when a custom `resolve.extensions` is set

_Found running the official Next.js e2e test suite against vinext (vinext 1.0.0-beta.8, Next.js 16.2). Error line is from the suite's own output._

**Fixture:** `test/e2e/app-dir/resolve-extensions`

**Symptom (build):** `[UNRESOLVED_ENTRY] nitro/dist/presets/bun/runtime/bun`. A fixture-level custom `resolve.extensions` appears to shadow the nitro bun preset's own entry resolution, so the preset runtime entry no longer resolves.

**Repro:** an app that overrides `resolve.extensions`, built through the nitro `preset: 'bun'`.

Start with the test/e2e/app-dir/resolve-extensions fixture and run its build through the nitro preset: 'bun'. Trace how the fixture-level resolve.extensions affects the bun runtime entry, then confirm the fixture builds without the [UNRESOLVED_ENTRY] error and the official e2e test passes.

Written by the indexing model from the issue text.