test(fire): cover dask+cupy dispatch and metadata preservation

[brendancol]

What

Every function in the fire module registers a dask+cupy backend in its ArrayTypeFunctionMapping, but only dnbr had a test that actually ran that path. The other six (rdnbr, burn_severity_class, fireline_intensity, flame_length, rate_of_spread, kbdi) exercised their dask+cupy dispatch with zero coverage. Only dnbr also asserted that input attrs, coords, and dims survive the call.

This adds, for each of the six:

• test_numpy_equals_dask_cupy: numpy/dask+cupy parity, matching the existing dnbr pattern
• test_output_preserves_metadata: via general_output_checks

Verification

All 73 test_fire.py tests pass on a CUDA host (cupy 13.6, numba CUDA available), including the 35 backend/metadata/dtype tests, none skipped. The branch is merged up to current main, so this composes with the recently merged #3396 (dask+numpy dtype fix) and #3397 (fire.py style cleanup).

Tests only, no source change. Found by the test-coverage sweep.

Gap found, not fixed (out of scope)

Inf-input coverage is still missing across the module. Low severity: these are pure per-cell math ops, so an Inf flows through the arithmetic rather than hitting a kernel-boundary path. Left for a follow-up rather than bundled here.

[brendancol]

Sweep audit summary (test-coverage, module fire):

• HIGH (Cat 1, backend coverage): dask+cupy dispatch registered for all 7 functions but invoked by a test only for dnbr. Added test_numpy_equals_dask_cupy for the other 6.
• MEDIUM (Cat 5, metadata preservation): only dnbr asserted attrs/coords/dims survive. Added test_output_preserves_metadata for the other 6.
• LOW (Cat 2, Inf inputs): untested, left out of scope (pure per-cell math, no kernel-boundary path).
• Not flagged: Cat 3 geometric degeneracies (no neighborhood/kernel, so 1x1/Nx1 add nothing); Cat 4 parameter coverage already strong (all 13 fuel models + invalid-range raises; heat_content/annual_precip covered).

All 73 test_fire.py tests green on a CUDA host, 35 backend/metadata/dtype tests run not skipped. No source change.