geotiff: reject tiled=False for VRT output and validate tile_size (#1862)

[brendancol]

Summary

• to_geotiff(..., '.vrt', tiled=False, ...) now raises ValueError up front. VRT output is always tiled, so a strip-mode request was never honored anyway.
• tile_size is validated unconditionally for .vrt paths, so tile_size=0 no longer reaches _write_vrt_tiled and triggers ZeroDivisionError.
• Closes geotiff: to_geotiff('.vrt', tiled=False, tile_size=0) raises ZeroDivisionError #1862.

Test plan

• tiled=False with a .vrt path raises ValueError.
• tile_size=0 with .vrt raises ValueError from _validate_tile_size (no ZeroDivisionError).
• Default args still succeed for .vrt.
• Existing VRT writer tests still pass.

[Copilot]

Pull request overview

This PR fixes issue #1862 where to_geotiff(..., '.vrt', tiled=False, tile_size=0) produced a ZeroDivisionError deep inside _write_vrt_tiled instead of a clean user-facing error. The fix takes both options suggested in the issue: reject tiled=False for .vrt paths up front, and ensure tile_size is validated on the VRT branch.

Changes:

• Compute _is_vrt_path once and suppress the misleading "tile_size ignored" warning on VRT paths.
• Raise ValueError for tiled=False on .vrt paths and re-validate tile_size inside the VRT branch.
• Add regression tests covering the rejected/validated combinations and a default-args sanity write.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
xrspatial/geotiff/init.py	Adds VRT-path detection, early tiled=False rejection, and explicit tile_size validation on the VRT branch.
xrspatial/geotiff/tests/test_to_geotiff_vrt_tiled_validation_1862.py	New regression tests verifying the new guards and ensuring default VRT writes still succeed.

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