FIX: Make quarterly.csv development dates ISO 8601-compatible.

[genedan]

Summary of Changes

2nd attempt at fixing the warnings for cl.load_sample('quarterly'). Simply changes the development dates to be ISO 8601-compatible, i.e, 1995-04 instead of 1995Q2.

Related GitHub Issue(s)

#840
#928

Additional Context for Reviewers

I decided to keep the test introduced in #929 because that ValueError block was uncovered and needed a test anyway:

chainladder-python/chainladder/core/base.py

Lines 408 to 412 in 31b3432

	if datetime_mapping is None:
	raise ValueError(
	"Unable to infer datetime for field(s): " + str(fields) +
	". Please check the underlying data or any supplied format arguments."
	)

Also made a change to the triangle docstring data argument which was causing typing warnings.

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

---

Note

Low Risk
Scope is sample CSV formatting, a docstring type hint, and a new regression test; no changes to core Triangle parsing logic beyond what existing _to_datetime already does.

Overview
Updates the quarterly sample dataset so development dates use ISO-friendly strings (1995-01, 1995-04, …) instead of 1995Q1-style labels, eliminating pandas datetime-inference warnings when calling cl.load_sample('quarterly'). Numeric cells in the CSV are unchanged aside from dropping trailing .0 on integers.

Also documents the Triangle constructor data parameter with a modern union type (DataFrame | DataFrameXchg | dict), and adds test_to_datetime_uninferrable_format_raises to assert a ValueError with message "Unable to infer datetime" when origin/development strings cannot be parsed.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.30%. Comparing base (31b3432) to head (a708691).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #930      +/-   ##
==========================================
+ Coverage   88.28%   88.30%   +0.01%     
==========================================
  Files          88       88              
  Lines        5029     5029              
  Branches      642      642              
==========================================
+ Hits         4440     4441       +1     
+ Misses        444      443       -1     
  Partials      145      145              

Flag	Coverage Δ
unittests	88.30% <ø> (+0.01%)	⬆️

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