fix: add validation for multi-character separators in split() (Closes #14649)

Co-authored-by: kuishou68 <54054995+kuishou68@users.noreply.github.com>
Co-authored-by: pojian68 <232320289+pojian68@users.noreply.github.com>
Signed-off-by: lingxiu58 <86288566+lingxiu58@users.noreply.github.com>

Author: kuishou68

strings/split.py

@@ -17,8 +17,21 @@ def split(string: str, separator: str = " ") -> list:
 
     >>> split(";abbb;;c;", separator=';')
     ['', 'abbb', '', 'c', '']
+
+    >>> split("a--b--c", separator="--")
+    Traceback (most recent call last):
+        ...
+    ValueError: separator must be a single character
+
+    >>> split("hello world", separator="")
+    Traceback (most recent call last):
+        ...
+    ValueError: separator must be a single character
     """
 
+    if len(separator) != 1:
+        raise ValueError("separator must be a single character")
+
     split_words = []
 
     last_index = 0