fix: always lower case the cache key

[wolfv]

Description

This should fix #5040 where the casing was inconsistent on macOS.

How Has This Been Tested?

I'll test this tomorrow with a git repository with upper cases.

AI Disclosure

• All code written by claude

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added sufficient tests to cover my changes.
• I have verified that changes that would impact the JSON schema have been made in schema/model.py.

[wolfv]

I've been looking at a test case but couldn't really produce one reliably just yet.