Add Ruff rule N802 (invalid-function-name)

[Abhayindia]

Summary of Changes

Adds the N802 rule (invalid-function-name).

Per the issue, the test-suite functions are renamed: test_OXDX_triangle -> test_oxdx_triangle, test_IBNR_methods -> test_ibnr_methods. Touching test_incremental.py pulls the whole file into the check, so it also drops an unused local and gets ruff format. test_1D_nanquantile in test_sparse.py is left for now - that file has ~70 other ruff violations, so its rename belongs with a full cleanup of it.

The codebase functions are excluded rather than renamed, for the "good reason" you mentioned:

• _prep_X_ml, _check_X, validate_X use scikit-learn's uppercase-X design-matrix convention (validate_X and _prep_X_ml are also called across modules)
• _G, G_ mirror Clark 2003's growth-function G (G_ appears in the ClarkLDF docstring's doctest)
• _get_MCL_model / _get_MCL_resids / _get_MCL_lambda (Munich Chain Ladder), _fit_OLS / _fit_OLS_thru_orig, PTF_formula, pZlower are internal helpers using standard statistical abbreviations

chapter_7_part_2.ipynb's ExNShtN exhibit functions are excluded too (that file also overlaps with #1223).

Related GitHub Issue(s)

Closes #1237

Additional Context for Reviewers

ruff check / ruff format with N802 is clean project-wide apart from the grandfathered files. The two renamed tests pass locally.

[github-actions]

Pyright Type Completeness

View the full pyright --verifytypes output for this commit

Project (full chainladder package, at this PR's head): 15.2% of exported symbols fully typed (209 / 1377)

	Known	Ambiguous	Unknown	Total
Project (head)	209	111	1057	1377

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): 0.0% fully typed (0 / 2); 2 no longer exported

	Known	Ambiguous	Unknown	Total
Patch	0	0	2	2

Patch symbol details

Symbol	Status	Change
chainladder.core.tests.test_triangle.test_oxdx_triangle	❌ unknown	new
chainladder.development.tests.test_incremental.test_ibnr_methods	❌ unknown	new
chainladder.core.tests.test_triangle.test_OXDX_triangle	—	no longer exported (was ❌ unknown)
chainladder.development.tests.test_incremental.test_IBNR_methods	—	no longer exported (was ❌ unknown)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.70%. Comparing base (5f34bcd) to head (0f57cbf).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1263   +/-   ##
=======================================
  Coverage   91.70%   91.70%           
=======================================
  Files          93       93           
  Lines        5435     5435           
  Branches      699      699           
=======================================
  Hits         4984     4984           
  Misses        327      327           
  Partials      124      124           

Flag	Coverage Δ
unittests	91.70% <ø> (ø)

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[henrydingliu]

looks good. please resolve conflict