geotiff: isort import-ordering drift in _writers/eager.py

[brendancol]

The geotiff package has no flake8/isort gate in CI, so import-ordering drift can land unnoticed.

isort --check-only --diff (configured line_length=100 in setup.cfg) flags one production file:

• xrspatial/geotiff/_writers/eager.py — the from .._validation import (...) block wraps the names across lines in a layout that does not match isort's 100-column packing.

No other geotiff production file has flake8 or isort findings. This is a Cat 4 (isort) style finding, no behavioral change.

Found by the style sweep (/sweep-style, geotiff module).