refactor(core): extract duplicate drop validation to _validate_contig…

[priyam0k]

Summary of Changes

Extracted duplicate contiguous drop validation in TrianglePandas.drop() into private helper _validate_contiguous_drop(). Added unit test coverage for the helper.

Related GitHub Issue(s)

Closes #1267

Additional Context for Reviewers

Pure refactoring that preserves all existing behavior, exception types, and error messages.

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)

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Note

Low Risk
Behavior-preserving refactor of Triangle.drop() validation with added unit tests; no API or logic changes beyond deduplication.

Overview
Pulls the shared origin and development drop rules out of TrianglePandas.drop() into a static helper _validate_contiguous_drop, which checks missing labels (KeyError / errors='ignore'), enforces edge-only contiguous drops (ValueError on interior gaps), and returns the boolean keep mask. drop() on those axes now delegates to the helper; behavior and messages stay the same.

Adds test_validate_contiguous_drop_helper to exercise the helper directly (valid first/last drops, missing labels, interior drop). The rest of the diff is mostly formatting in pandas.py and test style tweaks.

^{Reviewed by Cursor Bugbot for commit eb84f3a. Bugbot is set up for automated code reviews on this repo. Configure here.}

[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 / 1382)

	Known	Ambiguous	Unknown	Total
Project (head)	213	111	1058	1382

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 / 1)

	Known	Ambiguous	Unknown	Total
Patch	0	0	1	1

Patch symbol details

Symbol	Status	Change
chainladder.core.tests.test_triangle.test_validate_contiguous_drop_helper	❌ unknown	new

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.67%. Comparing base (670e86a) to head (eb84f3a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1286      +/-   ##
==========================================
- Coverage   91.68%   91.67%   -0.01%     
==========================================
  Files          93       93              
  Lines        5447     5443       -4     
  Branches      704      702       -2     
==========================================
- Hits         4994     4990       -4     
  Misses        328      328              
  Partials      125      125              

Flag	Coverage Δ
unittests	91.67% <100.00%> (-0.01%)	⬇️

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[henrydingliu]

looks good! do the ruff fixes allow any deletion of per-file ignore in pyproject.toml?

[priyam0k]

No, neither pandas.py nor test_triangle.py had entries in per-file-ignores. The commit only applied ruff format to satisfy the CI format check on the touched files.