fix: raise ValueError for multi-character separator in split

[SAY-5]

Describe your change:

strings/split.py previously iterated character by character comparing each char to separator, so any multi-character separator silently failed to match and the whole string was returned as a single element.

Now split validates len(separator) == 1 up front and raises ValueError("separator must be a single character") when it isn't, matching the actual capability of the implementation. Added a doctest covering the new error path.

Fixes #14649

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