geotiff: extract input validators into _validation.py (#1813 step 4) #1882

Reason

Five validators in xrspatial/geotiff/__init__.py (_validate_3d_writer_dims L114, _validate_dtype_cast L316, _validate_tile_size L1105, _validate_chunks_arg L1968, _validate_tile_size_arg L2022) are pure leaves over numpy types and string constants. They are called from to_geotiff, read_geotiff_dask, and read_geotiff_gpu; consolidating them lets later backend extractions stay focused on backend logic.

Proposal

Move them to xrspatial/geotiff/_validation.py. Re-import from __init__.py for the entry points still inline. Existing validator-specific tests (test_band_validation_1673.py, test_geotiff_band_bool_rejection_1786.py, the tile-size tests) continue to exercise behaviour through the public surface; the #1813 parity matrix is the regression backstop.

Refs: #1813.