Add regression doctests for KMP search and failure arrays

[Miladkhoshdel]

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 regression doctests for empty-pattern handling in knuth_morris_pratt() and examples for get_failure_array().

The search tests verify that an empty pattern returns 0 for both empty and nonempty text, consistent with str.find(). The helper tests cover distinct characters and overlapping prefixes.

This PR changes only doctests in one algorithm file.

Dependency: #15545 must merge first. Without that fix, the two empty-pattern doctests fail. After it merges, this branch should be updated before running CI again.

Validation:

• Both new get_failure_array() doctests pass.
• All 6 doctests pass with the pending fix applied in memory.
• Ruff lint and formatting checks pass.
• git diff --check passes.

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. — No new files added.
• 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. — Both functions now have doctests; passing CI depends on merging Fix empty-pattern handling in KMP search #15545 first.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation. — No new algorithms added.
• 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". — No linked issues to close.