geotiff: drop decorative post-write header check in writer

[brendancol]

Closes #1844.

Summary

• Removes the "post-write validation" block from write_geotiff and the streaming writer in xrspatial/geotiff/_writer.py.
• Both copies reparsed the first 16 bytes after the file (or temp file) was already on disk and emitted a Written file may be corrupt warning on failure.

Why

• The check only covered the TIFF / BigTIFF header that our own builder produced, so it said nothing about IFDs, tile or strip offsets and byte counts, geotags, COG layout, or whether any tile decodes.
• It ran after _write_bytes (or after the temp file was fully written), so even a real failure would have left a bad file on disk; the warning misled callers into thinking the file had been verified.
• The TIFF assembly is internal and type-checked. A genuine round-trip verifier would duplicate the reader and is better left to callers who can simply call read_geotiff(path).

What callers gain or lose

• Gain: clearer semantics. The writer no longer claims a verification it does not actually perform.
• Lose: nothing meaningful. No test asserted on this warning, and the parse of 16 self-produced bytes was effectively always succeeding.

Test plan

• pytest xrspatial/geotiff/tests/test_writer.py xrspatial/geotiff/tests/test_writer_matrix.py xrspatial/geotiff/tests/test_streaming_write.py xrspatial/geotiff/tests/test_parallel_writer_1800.py -- 145 passed.
• Full pytest xrspatial/geotiff/tests/ matches main: the remaining failures (test_features.py::TestPalette, test_predictor2_big_endian_gpu_1517.py, test_size_param_validation_gpu_vrt_1776.py) reproduce on main and are unrelated to this change.

[Copilot]

Pull request overview

This PR removes the “post-write validation” blocks in the GeoTIFF writers that reparsed the first 16 bytes (TIFF/BigTIFF header) after the file had already been written, and emitted a potentially misleading Written file may be corrupt warning.

Changes:

• Removed the post-write header parse/warn block from the eager write() path.
• Removed the equivalent post-write header parse/warn block from write_streaming() (temp-file path before os.replace).

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