Fix write_geotiff_gpu NaN-to-sentinel substitution (#1599)

The GPU writer (write_geotiff_gpu / to_geotiff with gpu=True) emitted
raw NaN bytes for missing pixels even when nodata=<finite> was
supplied, while the CPU writer substituted NaN with the sentinel
before encoding. xrspatial-only round-trips were unaffected because
the reader masks both NaN and the sentinel, but external readers that
mask only on the GDAL_NODATA tag (rasterio, GDAL, QGIS) treated NaN
pixels as valid data. rasterio reported 100% valid pixels on a GPU
file with 25 NaN inputs vs the CPU file's 25-invalid count.

Mirror the CPU writer's NaN-to-sentinel rewrite on the CuPy array
before compression. Gate on float dtype and finite nodata. Copy
defensively before mutating so a caller-owned CuPy buffer is not
modified, matching the CPU writer's arr.copy() at the equivalent step.

Add test_gpu_writer_nan_sentinel_1599.py: 7 regression tests covering
sentinel substitution, CPU/GPU byte equivalence, caller buffer
preservation, no-NaN no-op, NaN sentinel skip, rasterio-visible mask
parity, and 3D multiband substitution.

Discovered during the 2026-05-11 geotiff accuracy sweep.

Author: brendancol