Ensure overlay mounts create required directories

[Ndukiye]

Proposed Commit Message

fix(mounts): ensure overlay directories are created

Ensure required upper and work directories exist before overlay
mounts are applied. This prevents mount failures when directories
are missing during fstab processing.

Fixes #2916
LP: #1695277

## Additional Context
This change ensures cloud-init safely creates required directories
before attempting overlay filesystem mounts, improving robustness
on first boot scenarios.

## Test Steps
Run unit tests:
tox -e py3

Specifically:
pytest tests/unittests/config/test_cc_mounts.py -v

## Merge type

- [x] Squash merge using "Proposed Commit Message"
- [ ] Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)

[Ndukiye]

Hi, just following up on this PR. I’d appreciate any feedback or review when someone from the maintainers has time. Happy to make any changes needed.
Thanks!

[holmanb]

@Ndukiye Do you plan to continue working on this?

[Ndukiye]

@holmanb Yep! I addressed the feedback from your previous review by updating the documentation and pushed those changes a while ago. Since then I've just been waiting for another review. I'm happy to make any additional changes if you spot anything else.

[holmanb]

Please address the failing CI jobs.

[Ndukiye]

Done.
Fixed the lint issue (whitespace in my test). The unit test and labeler failures were already fixed on main (#6908, #6919), so I merged main to pick those up.