fix: raise ValueError in encode() for non-lowercase input

encode() previously accepted uppercase letters, digits, and other
non-lowercase characters silently, producing incorrect/out-of-range
values (e.g. negative numbers for uppercase letters) instead of
failing. Add input validation using str.islower() and str.isalpha()
to raise a ValueError when the input isn't purely lowercase a-z.
Added a doctest covering the new error case.

Author: alisatwat3

ciphers/a1z26.py

@@ -13,7 +13,11 @@ def encode(plain: str) -> list[int]:
     """
     >>> encode("myname")
     [13, 25, 14, 1, 13, 5]
+    >>> encode("ABCD")
+    ValueError: plain must contain only lowercase letters (a-z)
     """
+    if not plain.islower() or not plain.isalpha():
+        raise ValueError("plain must contain only lowercase letters (a-z)")
     return [ord(elem) - 96 for elem in plain]