sorts: type shell_sort for comparable items and add tests

[HarshRajSinghania]

Summary

Update shell_sort so it is typed and tested for any mutually comparable items, not only list[int].

Part of #15234

Motivation

Issue #15234 asks for one comparison sort per PR:

• switch list[int] to a Comparable protocol + type parameter
• add succeed cases for non-int comparables
• assert TypeError when items are not comparable

shell_sort.py was still unchecked and unclaimed.

Implementation

• Annotate shell_sort as shell_sort[T: Comparable](collection: MutableSequence[T]) -> MutableSequence[T] using the same Comparable protocol as insertion_sort / selection_sort.
• Leave the Marcin Ciura gap-sequence implementation unchanged.
• Add doctests for strings, mixed int/float values, and a TypeError on [1, "a"].
• Include shell_sort in test_sort_rejects_non_comparable_items in tests/test_sorts.py. It was already in the shared SORTS battery.

Testing

Ran locally against current master:

python3 -m doctest -v sorts/shell_sort.py
# 8 passed

python3 -m pytest tests/test_sorts.py -k "shell_sort or rejects" -q
# 22 passed, 210 deselected

[algorithms-keeper]

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