Add test for cost_distance dask all-impassable early return (#3367)

[brendancol]

Closes #3367

What

Adds one test (test_dask_all_impassable_friction_returns_nan) covering the f_min <= 0 early return in _cost_distance_dask. When friction has no passable cell and max_cost is finite, that branch short-circuits to da.full(..., nan) and keeps the input chunking, instead of running map_overlap or the iterative tile Dijkstra. Nothing reached this branch before.

The test builds an all-zero-friction dask+numpy raster with max_cost=5.0 and asserts the result is all-NaN, dask-backed, and keeps more than one partition.

Backend coverage

Test-only change, dask+numpy path. Source untouched.

Test plan

• test_dask_all_impassable_friction_returns_nan passes
• -W error::UserWarning confirms the early return is taken (no "iterative tile Dijkstra" warning)
• Full test_cost_distance.py green on a CUDA host (85 passed)

[brendancol]

PR Review: Add test for cost_distance dask all-impassable early return (#3367)

Blockers

None.

Suggestions

None.

Nits

None.

What looks good

• The test hits the intended branch. All-zero friction makes da.where(friction_da > 0, friction_da, np.inf) all-inf, so da.nanmin returns inf, np.isfinite(f_min) is False, and _cost_distance_dask takes the da.full(..., np.nan) early return at cost_distance.py:1150 instead of map_overlap or the iterative tile Dijkstra.
• The three assertions match that branch's contract: all-NaN output, dask-backed result, npartitions > 1 (chunking preserved).
• The PR proves the early return is reached via -W error::UserWarning. The iterative path raises a UserWarning, so its absence pins the branch and stops the test from silently drifting onto the iterative path later.
• Test-only change, source untouched. No backend-parity or numerical-correctness risk.

Checklist

• NaN handling correct: yes (asserts all-NaN)
• Edge case covered: yes (adds the missing all-impassable dask case)
• Dask chunking handled: yes (asserts npartitions > 1)
• No premature materialization: yes (single _compute at the end)
• Benchmark: not needed (test-only)
• Docstring present: yes (explains the branch)