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

[aoright]

Describe your change:

This PR adds input validation, lowercase input support, and replaces bypassable assertions in conversions/excel_title_to_column.py:

• excel_title_to_column: Replaces the bypassable assert column_title.isupper() with proper input validation raising ValueError for empty or non-alphabetic inputs. Added lowercase title support (e.g. "ab" -> 28).

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER"