Improve min_cost_string_conversion with validation, case-insensitive option, and more tests

[shreyasbp317]

This PR enhances the min_cost_string_conversion.py algorithm by adding input validation, case-insensitive mode, additional doctests, and improved CLI usability while preserving backward compatibility and passing all existing tests.

Describe your change:

• I made a few enhancements to the min_cost_string_conversion module:

• Added checks to handle invalid input more gracefully.

• Introduced an ignore_case option for case-insensitive string comparisons.

• Added more doctests to cover edge cases, single-character changes, and mixed-case scenarios.

• Cleaned up comments to make the code easier to follow.

• before:

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• after:
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• 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".