feat(conversions): add input validation and lowercase support to excel_title_to_column (#14872)

* feat(conversions): add input validation and lowercase support to excel_title_to_column

* style(conversions): shorten ValueError message to comply with ruff E501 line length limit

* Apply suggestion from @cclauss

---------

Co-authored-by: 辰言 <oncwnuIWp30GguOyJ615Fqj8H-yc@git.weixin.qq.com>
Co-authored-by: Christian Clauss <cclauss@me.com>

Author: 3 people

conversions/excel_title_to_column.py

@@ -12,8 +12,24 @@ def excel_title_to_column(column_title: str) -> int:
     28
     >>> excel_title_to_column("Z")
     26
+    >>> excel_title_to_column("a")
+    1
+    >>> excel_title_to_column("ab")
+    28
+    >>> excel_title_to_column("")
+    Traceback (most recent call last):
+        ...
+    ValueError: Column title must contain only alphabetic characters.
+    >>> excel_title_to_column("A1")
+    Traceback (most recent call last):
+        ...
+    ValueError: Column title must contain only alphabetic characters.
     """
-    assert column_title.isupper()
+    if not column_title or not column_title.isalpha():
+        msg = "Column title must contain only alphabetic characters."
+        raise ValueError(msg)
+
+    column_title = column_title.upper()
     answer = 0
     index = len(column_title) - 1
     power = 0