Keep dynamic asset prefixes outside configuration (#99490)
## Summary
Keep custom-server asset prefix changes in server render state, with
rendering and routing reading the same runtime value. Setting and
resetting the prefix continues to work without modifying or copying the
loaded config. This supports the broader effort to make Next.js
configuration immutable for adapters.
Adds a public custom-server test for App Router asset URLs, including
reset to an empty prefix.
## Verification
- Existing Pages custom-server dynamic-prefix tests pass in production
with Webpack and development with Turbopack (24 cases each).
- The production custom-server suite passes with Webpack; the new App
Router case also passes with Turbopack.
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