Add tests for all-NaN auto-levels RuntimeWarning regression (#2795)

[Melissari1997]

Closes #2795.

• Add test coverage for contours() on all-NaN rasters with auto level selection, verifying no RuntimeWarning is emitted.
• Tests cover numpy, dask+numpy, cupy, and dask+cupy backends.

The warning suppression was already in place (commit 58a44c3), but lacked explicit test coverage to prevent regression.

Backend coverage:

• numpy: tested
• dask+numpy: tested
• cupy: tested (skipped when cupy unavailable)
• dask+cupy: tested (skipped when cupy unavailable)

Test checklist:

• All-NaN numpy raster with auto levels returns empty, no RuntimeWarning
• All-NaN dask raster with auto levels returns empty, no RuntimeWarning
• All-NaN cupy/dask+cupy tests included (skipped without GPU)
• Existing contour tests still pass

[Melissari1997]

Review: Add tests for all-NaN auto-levels RuntimeWarning regression (#2795)

Summary

Test-only PR adding regression coverage for contours() on all-NaN rasters with auto level selection. The warning suppression was already implemented in commit 58a44c3; this PR ensures it does not regress.

Findings

No Blockers.

Suggestions:

• None.

Nits:

• The import warnings is repeated inside each test method. Consider moving it to the module-level imports at the top of the file to avoid redundancy across the four new tests.

Assessment

• Test structure follows existing patterns in TestNaNHandling
• Backend coverage is complete (numpy, dask+numpy, cupy, dask+cupy)
• All tests pass in the local environment
• PR correctly targets the upstream repo

[Melissari1997]

Follow-up review: Address review nit (#2795)

Moved import warnings to module-level imports. All four new tests now share the single module-level import. No functional changes.

Assessment

• Nit addressed
• Tests still pass
• Ready for merge