fix: reject multi-character separators in split() (Closes #14649)

[kuishou68]

Describe your change

This PR fixes the bug where the custom split function in strings/split.py silently returns the unsplit string when a multi-character separator is provided.

The function was comparing each individual character against the full separator string, so multi-character separators like "--" were never matched.

Fix: Added validation at the beginning of the function to raise a ValueError when separator is not exactly one character. This matches the expected behavior described in the issue.

Closes #14649

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests?
• Documentation change?

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.
• 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.
• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Closes split silently returns wrong result for multi-character separators #14649".