VRT missing-source handling returns partial mosaics by default #1799

Describe the bug

VRT source read failures are skipped by default. The reader warns and records attrs['vrt_holes'], but still returns a partial mosaic. That behavior is dangerous for production pipelines because a downstream caller can easily miss warnings and treat zero-filled or partially missing areas as valid data.

Expected behavior

The VRT reader should provide a public missing-source policy, such as missing_sources='raise'|'warn', and should strongly consider raising by default for unreadable sources.

Screenshots

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Smartphone

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Additional context

Strict mode already exists via XRSPATIAL_GEOTIFF_STRICT=1, but this behavior needs an explicit API-level contract so callers can choose safely without relying on environment variables.