geotiff: skip degenerate streaming tests when tifffile is unavailable (#1926)

The dask streaming degenerate-shape tests added in #1912 use a
_read_raw_pixels helper that does a bare ``import tifffile`` to verify
on-disk encoding. ``tifffile`` is an optional CI dependency, so when it
is not installed the three tests that call this helper error out with
ModuleNotFoundError instead of skipping. CI on main has been failing
since 15882e6 landed.

Other tests in the repo handle this with pytest.importorskip; this
change applies the same pattern so the three tests skip cleanly when
tifffile is missing.

Author: brendancol

xrspatial/geotiff/tests/test_dask_streaming_write_degenerate_2026_05_15.py

@@ -59,7 +59,7 @@ def _read_raw_pixels(path: str) -> np.ndarray:
     correctly. ``tifffile`` decodes the pixels but does not consult
     ``GDAL_NODATA``, so a raw read surfaces what is actually on disk.
     """
-    import tifffile
+    tifffile = pytest.importorskip("tifffile")
 
     with tifffile.TiffFile(path) as tif:
         return tif.asarray()