contours: handle infinite nan coords

[Melissari1997]

Summary

Closes #2704
This PR fixes the contour generation to properly handle input data containing infinite values. The marching squares algorithm now skips quadrilaterals with non-finite corners instead of producing invalid output or raising errors.

Key change:

• contour: Skip quads with any non-finite (NaN, +inf, -inf) corner values during contour line generation

AI-assisted review

• I ran the xarray-spatial AI-assisted review workflow for this PR.
• I reviewed the AI output myself and made my own judgment about what to accept.
• I understand this change and can maintain it.
• This PR does not attribute authorship to an AI tool or service.

AI-assisted review notes

The xarray-spatial review workflow was run against this change.

Issues found and addressed:

• Added regression tests for contour handling of infinite values (np.inf, -np.inf)
• Verified no change to public API behavior
• Confirmed existing NaN handling remains correct

Open concerns:

• None identified

Tests

Added tests in test_contour.py covering:

• Contour generation with np.inf values in input arrays
• Contour generation with -np.inf values in input arrays
• Mixed finite and infinite value scenarios

[github-actions]

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