[FIX] Apply Ruff fixes to chainladder/utils.

[genedan]

Summary of Changes

• I made one change to a private method name: self._fit_OLS_thru_orig() - OLS to lowercase
• Made an exception to _check_X(X) since capital X is taken from scikit-learn.
• Exception also made for function PTF_formula since this is public-facing.

Related GitHub Issue(s)

#1216

Additional Context for Reviewers

Checklist

• I passed tests locally for both code (uv run pytest) and documentation changes (uv run --directory docs jb build . --builder=custom --custom-builder=doctest)

---

Note

Low Risk
Mostly style and lint; the CuPy/Dask exception narrowing slightly broadens fallback behavior when optional runtimes fail at probe time, which is generally safer than swallowing all errors.

Overview
Applies Ruff cleanup across chainladder/utils (formatting, quotes, spacing, isinstance checks) and drops the per-file Ruff ignores for those modules in pyproject.toml now that they comply.

Behavioral tweaks: CuPy and Dask backend probes now catch ImportError and RuntimeError instead of a bare except, so a broken GPU/CUDA or Dask runtime falls back to NumPy with the existing warning when that backend was requested. WeightedRegression private fit helpers are renamed to _fit_ols / _fit_ols_thru_orig (call sites updated in the same file). PatsyFormula._check_X is a @staticmethod with an explicit noqa for the sklearn-style X name; public PTF_formula keeps a noqa for naming. Unused sparse.sp imports are removed from cupy.py and triangle_weight.py; Dask expand_dims uses slices instead of l.

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[github-actions]

Pyright Type Completeness

View the full pyright --verifytypes output for this commit

Project (full chainladder package, at this PR's head): 15.4% of exported symbols fully typed (213 / 1381)

	Known	Ambiguous	Unknown	Total
Project (head)	213	111	1057	1381

Other symbols referenced but not exported by chainladder: 13

	Known	Ambiguous	Unknown	Total
Other (head)	3	1	9	13

Symbols without documentation:

• Functions without docstring: 325
• Functions without default param: 0
• Classes without docstring: 10

Patch (exported symbols added or changed by this PR): 100.0% fully typed (1 / 1); 1 no longer exported

	Known	Ambiguous	Unknown	Total
Patch	1	0	0	1

Patch symbol details

Symbol	Status	Change
chainladder.utils.tests.test_sparse.test_1d_nanquantile	✅ known	new
chainladder.utils.tests.test_sparse.test_1D_nanquantile	—	no longer exported (was ✅ known)

[codecov]

Codecov Report

❌ Patch coverage is 84.84848% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.67%. Comparing base (8a47051) to head (2054a10).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
chainladder/utils/dask.py	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1290      +/-   ##
==========================================
- Coverage   91.68%   91.67%   -0.01%     
==========================================
  Files          93       93              
  Lines        5445     5443       -2     
  Branches      704      704              
==========================================
- Hits         4992     4990       -2     
  Misses        328      328              
  Partials      125      125              

Flag	Coverage Δ
unittests	91.67% <84.84%> (-0.01%)	⬇️

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[genedan]

Not sure what's going on with that codecov patch, I don't think I changed the lines in any meaningful way. We should also not go out of our way to test the dask or cupy files since they will be unsupported soon.

[henrydingliu]

does #1984 need to be stacked underneath this for the test_util changes?