Intelligent determination of plotting country lines

[jwarner8]

Fixes #655

Contribution checklist

Aim to have all relevant checks ticked off before merging. See the developer's guide for more detail.

• [x] Documentation has been updated to reflect change.
• [x] New code has tests, and affected old tests have been updated.
• [x] All tests and CI checks pass.
• [x] Added an entry to the top of docs/source/changelog.rst
• [x] Conda lock files have been updated if dependencies have changed.
• [x] Attributed any Generative AI, such as GitHub Copilot, used in this PR.
• [x] Marked the PR as ready to review.

[github-actions[bot]]

[jwarner8]

Given this PR does not touch the functionality or computation of diagnostics, and merely toggles country border plotting off/on, I have not requested a science review and only technical review.