fixing triangle.full_expectation_

[henrydingliu]

Summary of Changes

Related GitHub Issue(s)

Additional Context for Reviewers

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

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Note

Low Risk
Small property fix in reserve projection helpers with added regression tests; no auth, security, or external API surface changes.

Overview
Fixes full_expectation_ so cumulative vs incremental handling matches full_triangle_: it uses X_.is_cumulative when the fitted estimator has X_, otherwise self.is_cumulative. That corrects full_expectation_ on fit_predict outputs (and similar objects) that are triangles without a separate X_.

Tests now assert full_expectation_ from fit vs fit_predict agree (cumulative and incremental), and numeric comparisons use the shared atol fixture instead of hard-coded 0.00001 thresholds.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.05%. Comparing base (449b5c1) to head (71a8348).

Additional details and impacted files

@@              Coverage Diff               @@
##           #882-full-exp     #883   +/-   ##
==============================================
  Coverage          87.04%   87.05%           
==============================================
  Files                 86       86           
  Lines               4986     4989    +3     
  Branches             646      647    +1     
==============================================
+ Hits                4340     4343    +3     
  Misses               456      456           
  Partials             190      190           

Flag	Coverage Δ
unittests	87.05% <100.00%> (+<0.01%)	⬆️

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