VRT tests: positive coverage for simple GeoTIFF mosaics (#2342 work item)

[brendancol]

Sub-task of #2342.

Simple GDAL VRT mosaics over GeoTIFF sources are listed as supported for both eager and dask reads. Right now that subset has implicit coverage through other VRT tests but no dedicated positive cases. This issue tracks adding them so the supported shape is pinned down by tests.

Test cases:

• 2x1 horizontal mosaic of two compatible GeoTIFF tiles. Assert values match the expected concatenated array, plus coords and attrs (CRS, transform, nodata).
• 2x2 mosaic of four compatible tiles. Same assertions.
• Windowed read where the request window maps cleanly into source windows.
• Dask read of the same mosaics with multiple chunks.
• At least one multi-band mosaic case.

Acceptance:

• Tests pass on numpy and dask+numpy paths.
• Tests assert values, coords, and attrs (not just shapes).