fix and enhance utils.model_diagnostics

[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)

---

Note

Medium Risk
Changes a widely used diagnostics helper and moves shared CDF alignment logic; Benktander now errors without sample_weight, which may break callers that relied on optional weights.

Overview
This PR refactors CDF/LDF vertical alignment into Triangle.align_pattern (pattern triangles only) and wires MethodBase._align_cdf through it, with a guard test when called on non-pattern triangles.

model_diagnostics is tightened and extended: it now accepts a fitted Triangle, MethodBase, or Pipeline, rejects inputs missing ultimate_ / ibnr_ / ldf_, builds diagnostics from cum_to_incr on the model triangle (or the passed Triangle), and when groupby is unset adds LDF and CDF columns via align_pattern. groupby aggregation for IBNR (and related paths) is adjusted so summarized diagnostics stay consistent with grouped fits.

Benktander fit and predict now require sample_weight, matching other exposure-based estimators. Tests consolidate predict / weight validation across estimators and add model_diagnostics error and groupby parity coverage.

^{Reviewed by Cursor Bugbot for commit e9d0640. Bugbot is set up for automated code reviews on this repo. Configure here.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.42%. Comparing base (25e1192) to head (e9d0640).

Additional details and impacted files

@@                  Coverage Diff                   @@
##           #839_model_summary     #913      +/-   ##
======================================================
+ Coverage               88.16%   88.42%   +0.26%     
======================================================
  Files                      87       87              
  Lines                    4952     4969      +17     
  Branches                  629      635       +6     
======================================================
+ Hits                     4366     4394      +28     
+ Misses                    443      436       -7     
+ Partials                  143      139       -4     

Flag	Coverage Δ
unittests	88.42% <100.00%> (+0.26%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 90c4c0a. Configure here.}