fix(typegen): normalize %5F to _ before private folder check in dev bundler

[sleitor]

Summary

When Turbopack is used in dev mode, it URL-encodes _ as %5F in directory names when reporting file paths to the dev bundler. The private folder exclusion check:

if (normalizedPageName.includes('/_')) continue

works correctly in webpack dev mode (literal _ in paths), but fails in Turbopack mode because '/%5Ffoo'.includes('/_') is false.

Root Cause

In setup-dev-bundler.ts, when collecting app routes and layouts for the typegen manifest, Turbopack-encoded paths like /%5Ffoo/layout bypass the private folder filter. This causes:

1. Layout routes in _-prefixed directories leak into dev types — e.g. app/_foo/layout.tsx appears in dev .next/dev/types/routes.d.ts as "/%5Ffoo" in LayoutRoutes
2. Type mismatch — the production build correctly excludes private folders via ignorePartFilter: (part) => part.startsWith('_') in collectAppFiles, so .next/types/routes.d.ts does NOT have "/%5Ffoo" or "/_foo". The dev validator imports both files and fails with:

   Type '"/%5Ffoo"' is not assignable to type 'LayoutRoutes'
   

Fix

Normalize %5F → _ in normalizedPageName before the private folder check, matching the production build's behavior.

-const normalizedPageName = normalizePathSep(pageName)
+// Normalize URL-encoded underscores (Turbopack encodes '_' as '%5F' in
+// directory names). This ensures private folders (directories starting
+// with '_') are correctly excluded from routing, matching the behavior
+// of the production build (which uses ignorePartFilter: part.startsWith('_')).
+const normalizedPageName = normalizePathSep(pageName).replace(/%5F/g, '_')

The same %5F → _ normalization pattern is already used at line 682 for app route/page collection in the same function, and in route-discovery.ts for the production build.

Testing

• Regression verified against the existing includes('/_') private folder filter
• ESLint + lint-staged pass ✅
• The fix only affects the normalizedPageName used for the private folder check and the layout route string — the internal bundler identifiers (in appPaths) are unaffected, as they're normalized separately at line 682

Fixes #93700