fix: raise ValueError for multi-char separator in split()

[mithunbabbira]

Summary

Fixes #14649 - split() silently returns wrong result for multi-character separators

Problem

The custom split() function compares each character in the input string with the separator. Because of that, a multi-character separator such as "--" is never matched.

Solution

Added validation to raise ValueError when separator length != 1, and simplified the implementation to use Python's built-in str.split().

Testing

• All existing doctests pass
• New doctest added for multi-char separator validation

Fixes #14649

[algorithms-keeper]

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