docs: add doctests for min_cost_string_conversion

[Akhila21-6]

Adds doctests for the min_cost_string_conversion.py algorithm to improve the project's test coverage. This contribution was inspired by the work being done in the now-closed issue #9943.

Describe your change:

[ ] Add an algorithm?
[ ] Fix a bug or typo in an existing algorithm?
[x] 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 new functions have doctests that pass the automated testing.
• If this pull request resolves 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".