geotiff: read_vrt default lenient mode hides source read failures as zero-filled holes

[brendancol]

Describe the bug

xrspatial/geotiff/_vrt.py:647 wraps the per-source read_to_array call in a try/except that, by default, warns once and continues:

except (OSError, ValueError, struct.error,) + _CODEC_DECODE_EXCEPTIONS as e:
    if _geotiff_strict_mode():
        raise
    warnings.warn(...)
    continue

A continuing read returns a mosaic with holes. For integer VRTs the hole is the dtype zero, which downstream functions cannot distinguish from real data unless the band has a nodata value set.

Expected behavior

Either invert the default to strict (lenient becomes opt-in via XRSPATIAL_GEOTIFF_STRICT=0), or surface a stronger signal than a warning, for example a metadata['vrt_holes'] entry on the returned DataArray that records which sources failed and where.

Additional context

The current opt-in is XRSPATIAL_GEOTIFF_STRICT=1. Documented behavior should match what a new user will hit on a corrupt mosaic.