fix: Add missing lock around files dictionary

[wimoy]

The files dictionary could be modified and resized by another thread. Without the lock, the read is not synchronized and can cause a KeyNotFoundException.

[Copilot]

Copilot reviewed 1 out of 1 changed files in this pull request and generated 1 comment.

[vbreuss]

Thanks, @wimoy

[github-actions]

This is addressed in release v22.0.14.