Drop cuml.thirdparty_adapters.check_array dependency

[Intron7]

cuml 26.06 removed check_array from cuml.thirdparty_adapters (rapidsai/cuml#8052), breaking the rapids-prerelease CI matrix. Replaces the two check_array_cuml(init_coords, dtype=np.float32, accept_sparse=False) call sites in _draw_graph.py and _umap.py with a small local _validate_init_pos helper in tools/_utils.py — same behavior (reject sparse, coerce to cupy float32, ensure 2D), no cuml-internal dependency.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.15%. Comparing base (2926263) to head (b004385).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/rapids_singlecell/tools/_utils.py	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #660      +/-   ##
==========================================
- Coverage   88.17%   88.15%   -0.02%     
==========================================
  Files          96       96              
  Lines        7060     7067       +7     
==========================================
+ Hits         6225     6230       +5     
- Misses        835      837       +2     

Files with missing lines	Coverage Δ
src/rapids_singlecell/tools/_draw_graph.py	97.29% <100.00%> (ø)
src/rapids_singlecell/tools/_umap.py	81.53% <100.00%> (-0.28%)	⬇️
src/rapids_singlecell/tools/_utils.py	78.23% <75.00%> (-0.19%)	⬇️