test: add palindrome edge-case doctests

[devam007]

Describe your change

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- No algorithm logic is changed.
• Documentation change?

Add six edge-case examples to strings/palindrome.py: empty input for the
four ordinary palindrome implementations, punctuation-only input for the
normalizing implementation, and a non-palindromic input containing digits.
These document existing behavior and protect against dropping digits during
normalization. This is a test-coverage improvement, not an algorithm bug fix.

Validation

Locally on Python 3.12.8:

python -B -m doctest -v strings/palindrome.py
python -B -m pytest strings/palindrome.py --doctest-modules -p no:cacheprovider -q
python -B -m ruff check --no-cache strings/palindrome.py
python -B -m ruff format --check --no-cache strings/palindrome.py
git diff --check

All 15 doctest examples pass, collected as five pytest items. Ruff checks and
the whitespace check pass. The full suite and upstream Python 3.15t environment
have not been run locally.

Checklist

• I have read CONTRIBUTING.md and AGENTS.md.
• This PR changes only one algorithm file and only its doctest examples.
• Existing and added examples pass the targeted local checks above.

Prepared with Codex AI assistance as a beginner contribution. No new algorithm,
independent-authorship claim, or upstream CI result is claimed.