test: exclude optional plotting extra from coverage and unit-test _compute_extent

[jsteinberg34]

Summary

This is a small coverage-reporting cleanup, not new test logic. After Phases 1–4, every module except plotting.py sits at 76–100% coverage, but the headline number was a misleading 64% — because plotting.py (an optional visualization extra whose matplotlib/cartopy dependencies are not installed in CI) was being measured at ~5% and single-handedly dragging the total down. This PR makes the reported number reflect the code CI actually exercises, and adds a small real test for the one piece of plotting.py that contains pure logic.

What changes

• pyproject.toml — exclude plotting.py from coverage measurement via omit = ["*/plotting.py"] under [tool.coverage.run], with a comment explaining why. plotting.py is an optional extra (pip install groundtrack[plotting]); its matplotlib/cartopy dependencies are intentionally not installed in CI, so its statements cannot be fairly measured there. The plot_* functions are thin cartopy/matplotlib wrappers that end in plt.show() — there is no meaningful logic to assert without brittle image-comparison tests, and a broken plot is immediately visible to the user rather than silently corrupting scientific output.
• tests/test_plotting.py — add 6 unit tests for _compute_extent, the only pure function in plotting.py (it computes the map bounding box from track/station coordinates using numpy + wrap_lon_deg, and imports fine without matplotlib/cartopy). Covers the global fallback, padding, custom pad, stations-only, combined track+stations, and longitude wrapping. So the omitted module is not literally untouched — its one piece of real logic is verified.

Effect on coverage

	Coverage
Before (with plotting.py)	64%
After (plotting.py omitted)	~87%

This is reporting only — no production code changed, and the omitted module's only logic is now tested. Every CI-exercised module remains at 76–100%.

Testing

pytest          # 118 passed, fully offline

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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