fix(files/type): preserve numeric keys (follow-up)

[joshtrichards]

• Expands on #50660 which was for #42902

Summary

Primary changes:

• Fixes implementation for custom mappings to preserve numeric keys (which was passing the prior unit test)
• Fixes test that wasn't testing the custom mappings code path

Secondary changes:

• Refactors tests to use an isolated temp dir with proper cleanup
• Adds a focused test to see if numeric-looking keys (e.g. "001") are preserved
  • (the current implementation is brittle and can break if we aren't careful with how we manage keys)

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)