geotiff: open_geotiff silently drops missing_sources when routing to read_vrt

[brendancol]

Summary

open_geotiff does not accept missing_sources and does not forward it to read_vrt when dispatching to a .vrt source. Callers who want strict failure on broken VRT sources must call read_vrt directly, defeating the documented dispatcher.

Repro

from xrspatial.geotiff import open_geotiff
open_geotiff("mosaic.vrt", missing_sources="raise")
# TypeError: open_geotiff() got an unexpected keyword argument 'missing_sources'

Why this matters

Same class of bug as #1561 / #1605 / #1685 / #1795: a backend kwarg is reachable on the routed-to function but not through the documented dispatcher. #1806 added missing_sources to read_vrt but did not propagate it to open_geotiff. The VRT dispatch branch in xrspatial/geotiff/__init__.py:813-815 builds the forwarded call without it:

return read_vrt(source, dtype=dtype, window=window, band=band,
                name=name, chunks=chunks, gpu=gpu,
                max_pixels=max_pixels)

Proposed fix

Add missing_sources='warn' to open_geotiff, forward it to read_vrt, and reject it when the source is not a VRT (TIF or file-like) with a clear ValueError, matching the pattern open_geotiff already uses for on_gpu_failure and overview_level. The default 'warn' preserves historic behaviour.

Found during the geotiff API consistency sweep (Cat 5, MEDIUM).