Disable sandbox mesh for K8S regions

[nicoche]

Summary

• Force sandbox deployment mesh disabled when KOYEB_K8S_REGION is present.
• Add mesh resolution tests, including explicit enable/disable and K8S region overrides.

Verification

• git diff --check
• uv run black koyeb/sandbox/utils.py koyeb/sandbox/test_utils.py
• uv run flake8 --ignore=E501,W503 koyeb/sandbox/utils.py koyeb/sandbox/test_utils.py
• uv run pytest koyeb/sandbox/test_utils.py
• python -m unittest koyeb.sandbox.test_utils

Notes

• Plain python -m black and python -m flake8 are not installed globally; checks were run through uv.
• Repository-wide flake8 reports pre-existing E501/W503 issues in the touched modules, so the targeted lint command ignores those rules and confirms no new undefined-name errors.