fix: correct typos 'occurence' and 'existance'

[thecaptain789]

Fixes two spelling errors in comments.

[cursor]

PR Summary

Low Risk
Comment-only spelling fixes; no runtime behavior or API changes.

Overview
Fixes two spelling typos in comments: existance→existence in modal/snapshot.py and occurence→occurrence in test/container_test.py.

^{Written by Cursor Bugbot for commit 29d8e32. This will update automatically on new commits. Configure here.}