Improve file filtering when using the frontend indexing

[jtpio]

The pattern matching logic uses simple string inclusion which may produce false positives. For example, excluding 'test' would also exclude 'contest.py'. Consider implementing proper glob pattern matching or at minimum word boundary matching to avoid unintended exclusions.

Originally posted by @Copilot in https://github.com/jupyterlab-contrib/jupyterlab-quickopen/pull/71#discussion_r2314297174