geotiff: add mask_nodata kwarg to preserve integer source dtypes (closes #2052)

[brendancol]

Closes #2052.

Problem

open_geotiff(path, dtype="uint16") on a uint16 file whose nodata
sentinel matched real pixels raised ValueError. The masking block
promoted the array to float64 before the dtype= cast ran, and the
cast then rejected the float-to-int conversion.

The docstring promised "Pass dtype=... to keep the source dtype",
but the most common integer-nodata case never reached the cast with
its source dtype intact.

Fix

Add mask_nodata: bool = True to open_geotiff, read_geotiff_dask,
read_geotiff_gpu, and read_vrt. When mask_nodata=False:

• skip the sentinel-to-NaN promotion;
• keep the source dtype (or honour dtype=);
• attrs['nodata'] still carries the raw sentinel so callers can
  mask themselves.

Default stays True, so existing callers see no change.

The eager reader's existing local named mask_nodata (the resolved
sentinel value used for comparison) is renamed to nodata_sentinel
to free up the kwarg name. Docstrings on all four readers document
the new kwarg and the dtype= interaction.

Threading

The kwarg flows through every public reader entry, including:

• open_geotiff dispatcher (forwards to all backends)
• read_geotiff_dask (gates the effective_dtype promotion and the
  per-chunk _delayed_read_window mask)
• read_geotiff_gpu (gates _apply_nodata_mask_gpu on the eager
  path; gates the chunked GDS declared_dtype calculation and the
  per-chunk task)
• read_vrt (gates _apply_integer_sentinel_mask on eager and
  chunked paths)

The pre-existing kwarg-order canonical-list test (#1935) picks up
the new entry.

Tests

10 new tests in test_mask_nodata_kwarg_2052.py:

• regression repro: dtype="uint16" raised before, now works with
  mask_nodata=False
• default mask_nodata=True still promotes to float64 + NaN
• no-match case: both modes return the same uint16 array
• float file: NaN nodata is a no-op either way
• dtype="uint32" integer-to-integer cast works with the opt-out
• dask path: integer source dtype survives with mask_nodata=False
• dask path: default still promotes
• dask path: dtype= + mask_nodata=False round-trip

Test plan

• pytest xrspatial/geotiff/tests/test_mask_nodata_kwarg_2052.py -x
• pytest xrspatial/geotiff/tests/ -k "nodata or dtype" -q
• Full geotiff sweep: only pre-existing failures
  (test_predictor2_big_endian_gpu_1517, test_size_param_validation_gpu_vrt_1776)
  remain, and both reproduce on main.