test: cover insertion sort stability with equal-priority items - #15480

Describe your change

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

Add a doctest demonstrating that insertion sort preserves the relative order
of equal-priority items. The example compares items by priority only, keeping
their labels distinguishable.

Only the docstring in sorts/insertion_sort.py changes.

Validation

• All 18 module doctests passed on Python 3.15t.
• Applicable repository pre-commit hooks and git diff --check passed.
• An in-memory mutation from < to <= reversed equal-priority items,
  confirming that the example detects loss of stability.
• The full repository test suite was not run.

AI assistance

ChatGPT helped explain stability and prepare the example.
I reviewed the diff and ran the reported checks locally.

Checklist

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues, then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".

Unchecked conditional items are not applicable: no new files or algorithms are added, and this PR does not resolve an existing issue.