Use TypeError for non-string input in count_vowels

[yaadhuu]

Describe your change:

• Fix a bug or typo in an existing algorithm?

This pull request updates count_vowels to raise TypeError for invalid
(non-string) inputs instead of using a generic exception. The algorithm
logic, behavior, and time complexity remain unchanged, and all existing
doctests continue to pass.

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.
• 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.