fix(tests): use assert in test_n_periods so failures actually fail

[SaguaroDev]

Summary of Changes

chainladder/development/tests/test_development.py::test_n_periods ended with return xp.all(...) instead of assert xp.all(...). Two consequences:

1. The boolean result was silently discarded, so the test did not actually fail when the equality broke — regressions in Development(n_periods=3, average='volume') were masked.
2. Pytest 9 emits PytestReturnNotNoneWarning for any test function that returns a non-None value.

This PR replaces the single return with assert. One-line change.

Verification

• Before fix, with -W error::pytest.PytestReturnNotNoneWarning: the test errors out on the warning.
• After fix: pytest chainladder/development/tests/test_development.py -W error::pytest.PytestReturnNotNoneWarning → 35 passed, 1 xfailed, 0 PytestReturnNotNoneWarnings.
• Mutation check: temporarily replaced one expected LDF (1.164 → 9.999) and confirmed the test now fails with AssertionError (it previously would have passed silently).

This is purely a pytest hygiene fix and is independent of the pandas 3.0 work in #664 / #729.

Related GitHub Issue(s)

Closes #743

Additional Context for Reviewers

I grepped ^ return across test_*.py to confirm this is the only test in the package with this bug; the other 5 matches are all legitimate fixture returns.

---

Note

Low Risk
Low risk: changes a single test to use assert, making failures visible and eliminating pytest warnings; no production code changes.

Overview
Fixes test_n_periods in chainladder/development/tests/test_development.py to assert xp.all(...) instead of returning the boolean result.

This ensures regressions in Development(n_periods=3, average="volume") cause an actual test failure and avoids pytest warnings about non-None test returns.

^{Reviewed by Cursor Bugbot for commit f1fd507. 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 85.94%. Comparing base (822c052) to head (f1fd507).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #744   +/-   ##
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  Coverage   85.94%   85.94%           
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  Files          85       85           
  Lines        4924     4924           
  Branches      637      637           
=======================================
  Hits         4232     4232           
  Misses        491      491           
  Partials      201      201           

Flag	Coverage Δ
unittests	85.94% <ø> (ø)

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[genedan]

Thanks for taking this on @SaguaroDev. We have a lot of warnings in pytest, so we really appreciate any help on knocking these out.

[kennethshsu]

@SaguaroDev amazing!! Well done on your first PR! Welcome to the group!

@HeatherLDavis should be reaching out to you shortly if she hasn't yet, in case you are interested in joining the WG calls (there's a bit more detail on the readme).