Fix unset crossOrigin in Turbopack manifests (#97164)
## Summary
Prevent Turbopack from adding `crossorigin=""` to generated script tags
when `crossOrigin` is not configured, and add production coverage for
static prerendering, dynamic rendering, and `output: 'export'`.
This regressed in #92070 when Turbopack added `crossOriginLoading`
support. The client reference manifest changed from an optional
cross-origin value to the shared `CrossOrigin` enum. Serde serialized
the enum's default `None` variant as the string `"none"`, so React saw a
present string and treated it as anonymous CORS, emitting
`crossorigin=""`. Webpack continued to omit its `undefined` value.
Skip serializing `CrossOrigin::None` in the Turbopack client reference
manifest so an unset option remains absent while preserving explicit
anonymous and credentialed modes.
## Verification
- `NEXT_TEST_PREFER_OFFLINE=1 pnpm test-start-turbo
test/e2e/app-dir/app-config-crossorigin/index.test.ts`
- `NEXT_TEST_PREFER_OFFLINE=1 pnpm test-start-webpack
test/e2e/app-dir/app-config-crossorigin/index.test.ts`
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