types(unknown_sort): constrain items to Comparable

[AuroraAeon]

Part of #15234

What

sorts/unknown_sort.py — bind merge_sort's element type to a Comparable Protocol so the signature states "a list of items that can be compared with each other" instead of a bare list, and keeps the element type in the return:

def merge_sort[T: Comparable](collection: list[T]) -> list[T]:

start/end are annotated list[T] so the element type survives the local bindings instead of degrading to list[Any].

Tests

Doctests added for:

• a comparable non-int type — ["banana", "apple", "cherry"] and [3.14, 1.5, 2.7]
• the failure mode — merge_sort([1, "a"]) raises TypeError rather than silently mis-sorting

tests/test_sorts.py:

• added to the shared SORTS battery (all 11 cases, including the Person and Dog dataclass cases)
• added to test_sort_rejects_non_comparable_items

Run with:

python -m pytest tests/test_sorts.py sorts/unknown_sort.py -q

→ 321 passed, 12 of them new. ruff check and ruff format --check are clean, and the wider sorts/ tier is unaffected (90 passed).

Note on the import alias

sorts/merge_sort.py already exports a merge_sort, so the import is aliased as unknown_merge_sort. The parametrize id therefore reads merge_sort0/merge_sort1 rather than the alias name — the same way shrink_shell_sort already shares the shell_sort id in this battery. No behaviour change; only annotations and doctests were added.

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[AuroraAeon]

Closing this one myself, for two reasons:

1. @cclauss raised the keep-vs-drop question upthread in sorts: make algorithms sort any comparable items, not just ints #15234. sorts/unknown_sort.py exports a function literally named merge_sort that is actually a double-ended min/max selection sort — it leans on min(), max() and list.remove() — so it duplicates selection_sort's idea under a misleading name.
2. That is a repo-hygiene decision that should not ride along inside a typing PR.

Happy to re-open this if the verdict is "keep the file" — the typing change itself is mechanical.