feat(conversions): add input validation and fix bugs in Roman Numerals and Excel Column Title conversions

[aoright]

This PR improves input validation, adds lowercase input support, and fixes boundary bugs across two conversion files:

1. roman_numerals.py:

   • roman_to_int: Seamlessly converts lowercase input strings (e.g. "iii" -> 3) and raises a proper ValueError with clear messages for empty or invalid inputs.
   • int_to_roman: Validates that the input is an integer in the range 1 <= number <= 3999. Raises ValueError for out-of-range/non-integer inputs instead of returning incorrect values (e.g., int_to_roman(-5) used to return "CMXCV").

2. excel_title_to_column.py:

   • excel_title_to_column: Replaces the bypassable assert column_title.isupper() with a proper input validation raising ValueError for empty or non-alphabetic inputs. Added lowercase title support.

[algorithms-keeper]

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