read_vrt chunks mode materializes the full mosaic before chunking #1798

Describe the bug

read_vrt(chunks=...) returns a dask-backed DataArray, but it first eagerly assembles the full VRT into memory and only then chunks the result. This makes the API look lazy while still carrying full eager memory risk for large mosaics.

Expected behavior

Either read_vrt(chunks=...) should be truly lazy, building dask tasks over VRT windows, or the public contract should be changed to make the eager materialization explicit.

Screenshots

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Desktop

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Smartphone

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Additional context

This affects large VRT mosaics and can produce out-of-memory failures before dask has a chance to help.