Fix route.replace crash with --debug-build-paths (vercel#90312)

## Summary

- Fixes `route.replace is not a function` error when running `next build
--debug-build-paths` on projects with parallel routes

**Root cause:** Turbopack filters routes at the route level (e.g.,
`/parallel-test` includes all page entries), while JS route discovery
filters at the file path level (e.g., only
`app/parallel-test/page.tsx`). This causes the `appPathsManifest` to
contain entries (like `/@sidebar/page`) that `mappedAppPages` doesn't
have. The unguarded lookup at
`mappedAppPages[originalPath].replace(...)` crashes on `undefined`.

**Fix:** Added a guard to skip manifest entries that don't exist in
`mappedAppPages`.

## Test plan

- [x] Verified `next build --debug-build-paths
'app/parallel-test/**/page.tsx'` succeeds on test fixture
- [x] Verified other `--debug-build-paths` patterns still work
(multi-page, pages-only, etc.)

Author: devjiwonchoi

packages/next/src/build/index.ts

@@ -2185,7 +2185,10 @@ export default async function build(
                   for (const [originalPath, normalizedPath] of Object.entries(
                     appPathRoutes
                   )) {
-                    if (normalizedPath === page) {
+                    if (
+                      normalizedPath === page &&
+                      mappedAppPages[originalPath]
+                    ) {
                       pagePath = mappedAppPages[originalPath].replace(
                         /^private-next-app-dir/,
                         ''