refactor(filesystem): clean up isValidPath() logic and update docs

[joshtrichards]

Summary

• Optimizations/code cleanup:
  • Removes redundant checks; normalization already guarantees path is absolute
  • Replaces strrchr with str_ends_with
  • Adds parameter and return type hints (were already enforced via static analysis; should be safe)
• Docs:
  • Note it's primary role as a security boundary for path validation and what specifically it handles

No functional change in logic: the production code path is just simpler and optimized.

TODO

• ...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)