Normalize encoded dynamic placeholders in app routes (#91603)
## Summary
- normalize encoded dynamic placeholders like `%5Bproject%5D` before
parsing app route segments
- add a fallback params regression test covering encoded placeholder
inputs
- assert vary-param segment prefetch responses do not contain encoded
root param placeholders
## Testing
- pnpm testheadless
packages/next/src/server/request/fallback-params.test.ts
- pnpm testheadless
test/e2e/app-dir/segment-cache/vary-params/vary-params.test.ts -t
"tracks root param access via rootParams API"
