Trace development route compilation (#96454)
## Summary
- Trace development route compilation through
`DevBundlerService.ensurePage` after route preparation selects a route.
- Present the existing development bundler work as `compile route` in
Request Insights so cold compilation can be distinguished from route
matching.
- Keep the span internal by default without expanding the default public
OpenTelemetry allowlist.
- Preserve compilation error propagation and the existing
route-preparation parentage.
Warm requests can still record the span when `ensurePage` runs, but the
work is normally negligible after the route is compiled.
## Verification
- `HEADLESS=true pnpm test-dev-turbo
test/development/app-dir/request-insights-route-preparation/request-insights-route-preparation.test.ts`
- `HEADLESS=true pnpm test-dev-webpack
test/development/app-dir/request-insights-route-preparation/request-insights-route-preparation.test.ts`
- Both bundlers passed 2/2 tests at the final stack head.
- `pnpm --filter=next build`
