[CHORE] Ruff + Test Fixtures for Tail tests

[henrydingliu]

Summary of Changes

ruff fixes
adding tail_sample as a test fixture
adding a test for TailClark to cover application of truncation_age

Related GitHub Issue(s)

#1355
#1361
#1277

AI/LLM Usage

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---

Note

Low Risk
Formatting and test-only changes; tail estimator behavior is unchanged aside from clearer concatenate kwargs.

Overview
Applies Ruff-driven cleanup across the chainladder/tails package: module/class/method docstrings are reformatted to the multi-line style, and several calls (notably xp.concatenate in TailClark) use explicit axis=-1 and expanded line breaks without changing tail logic.

Test infrastructure adds a shared tail_sample pytest fixture (with the same normal_run / sparse_only_run parametrization as other samples) and refactors Bondy, curve, and constant tail tests to use fixtures plus TYPE_CHECKING annotations instead of inline load_sample. A new test_truncation_age asserts that a very large truncation_age on TailClark matches omitting truncation on the final CDF.

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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): 15.0% of exported symbols fully typed (208 / 1383)

	Known	Ambiguous	Unknown	Total
Project (head)	208	111	1064	1383

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: 327
• 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.tails.tests.test_clark.test_truncation_age	❌ unknown	new

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.86%. Comparing base (01bf6bf) to head (c7433f0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1374      +/-   ##
==========================================
+ Coverage   91.78%   91.86%   +0.08%     
==========================================
  Files          96       97       +1     
  Lines        5488     5496       +8     
  Branches      707      707              
==========================================
+ Hits         5037     5049      +12     
+ Misses        327      325       -2     
+ Partials      124      122       -2     

Flag	Coverage Δ
unittests	91.86% <100.00%> (+0.08%)	⬆️

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