Fix `--debug-build-paths` bracket escaping for glob patterns (#88660)
Always escape Next.js dynamic route brackets (e.g., `[slug]`) in `--debug-build-paths` patterns instead of checking filesystem existence.
The previous approach failed when glob wildcards like `**` preceded bracket expressions (e.g., `app/**/[slug]/page.tsx`) because `fs.existsSync` cannot resolve paths containing glob characters. Since users of this flag are always referring to actual Next.js routes, brackets should always be treated as literal directory names rather than glob character classes.