style(pathfinding): drop unused has_dask_array import, fix isort ordering

[brendancol]

Closes #3650

Style sweep cleanup for xrspatial/pathfinding.py, flagged by the project's own tooling (setup.cfg: flake8 max-line-length=100, isort line_length=100).

• Cat 3 (F401): dropped has_dask_array from the xrspatial.utils import. It is never used in the module and nothing imports it from xrspatial.pathfinding, so it is a refactor leftover, not a re-export.
• Cat 4 (isort): ran isort xrspatial/pathfinding.py. Reorders import dask / import dask.array as da, swaps _heap_pop, _heap_push into alphabetical order, grid-wraps the xrspatial.utils block, and adds a blank line after the function-local import cupy.

Not included: the mutable default barriers: list = [] (Cat 5) is item 4 of #3644 and will be fixed there.

No behavioral change intended. Import edits are static and apply uniformly across all four backends.

Test plan:

• flake8 xrspatial/pathfinding.py clean
• isort --check-only xrspatial/pathfinding.py clean
• pytest xrspatial/tests/test_pathfinding.py — 57 passed (CUDA-capable box, cupy paths exercised)

Found by the /sweep-style deep sweep (2026-07-08).

https://claude.ai/code/session_0155N4QGamQVxgpAAPbpQNq4

[brendancol]

PR Review: style(pathfinding): drop unused has_dask_array import, fix isort ordering

Style-only change. I verified the two substantive claims against the head branch.

Blockers

None.

Suggestions

None.

Nits

None.

What looks good

• has_dask_array no longer appears anywhere in pathfinding.py, and nothing imports it from the module. Imports out of xrspatial.pathfinding are limited to a_star_search, multi_stop_search, _available_memory_bytes, and a few test-only helpers, so dropping it is a leftover cleanup, not a broken re-export.
• The six kept names in the xrspatial.utils block are all still referenced (ngjit 8x, has_cuda_and_cupy 5x, _validate_raster 5x, the rest 3x each), so the grid-wrap didn't strip anything live.
• The import dask / import dask.array as da reorder, the _heap_pop / _heap_push swap, and the blank line after the function-local import cupy are all cosmetic. No runtime behavior changes.
• flake8 xrspatial/pathfinding.py and isort --check-only xrspatial/pathfinding.py both come back clean on the branch.

Scoping the mutable-default barriers: list = [] out to #3644 is the right call; keeping this PR to F401 + isort keeps the diff reviewable.

Checklist

• Imports dropped are genuinely unused and not re-exported
• Kept imports all still referenced
• No behavioral change (cosmetic reordering only)
• flake8 and isort clean on the branch