[REFACTOR]: Continue work on cl.__init__.py refactor

[genedan]

Summary of Changes

• Move datetime defaults out of Options.
• Add validation to option getters and setters.
• Enable resetting of single options.
• Deprecate cl.array_backend() and cl.auto_sparse()

Related GitHub Issue(s)

#861
#858
#860
#859

Additional Context for Reviewers

It was hard to do this cleanly in multiple PRs, so I hope you can follow this. I found that aside from deprecating 2 functions, the other sub-items were intertwined.

• 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 affect global datetime handling and package-level configuration used across triangle valuation and utilities; removing deprecated helpers is a breaking API change for callers still using cl.array_backend() or cl.auto_sparse().

Overview
This refactor tightens package configuration in chainladder.__init__: Pandas-derived datetime dtype/unit are now module-level __dt64_dtype__ and __dt64_unit__ (for Pandas 2/3 compatibility), not Options fields. Options gains _validate_option on get/set/reset, a _defaults snapshot, and reset_option(option) to restore one setting or all defaults. cl.array_backend() and cl.auto_sparse() are removed in favor of chainladder.options.set_option.

Triangle valuation and date helpers in core/base.py, core/pandas.py, and utils/utility_functions.py now use the module constants instead of options.DT64_*. Tests were updated accordingly (including single-option reset and invalid-option errors).

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[codecov]

Codecov Report

❌ Patch coverage is 95.65217% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.17%. Comparing base (70fe2a5) to head (8030ac6).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
chainladder/__init__.py	94.11%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #867      +/-   ##
==========================================
+ Coverage   87.16%   87.17%   +0.01%     
==========================================
  Files          86       86              
  Lines        5047     5051       +4     
  Branches      648      650       +2     
==========================================
+ Hits         4399     4403       +4     
  Misses        457      457              
  Partials      191      191              

Flag	Coverage Δ
unittests	87.17% <95.65%> (+0.01%)	⬆️

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[henrydingliu]

_defaults maybe?
and do a hasattr check on _defaults to get rid of the bugbot comment?

[genedan]

I switched it to _defaults and then excluded names beginning with an underscore from the dict. Also did small work on #567, since I noticed there was nothing on top of the file. This is the overall package-level __init__.py so I did a generic description.

[cursor]

Cursor Bugbot has reviewed your changes and found 2 potential issues.

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