[huuan26]

Describe your change

• 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?

Add edge-case doctests to strings/is_isogram.py:

• Empty string input is_isogram("") -> True (vacuously true)
• Single character input is_isogram("a") -> True
• Mixed-case sensitivity test is_isogram("Alphabet") -> False (contains duplicate 'a'/'A')
• Longest standard English isogram is_isogram("subdermatoglyphic") -> True
• Strings containing whitespace is_isogram("hello world") -> ValueError
• Strings containing punctuation is_isogram("isogram!") -> ValueError

No algorithm logic was changed.

Checklist

• I have read CONTRIBUTING.md and AGENTS.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.