updates the Apriori prune() function to improve performance while preserving correctness.

[kdt523]

This PR updates the Apriori prune() function to improve performance while preserving correctness.
What’s included
Use a set for O(1) membership checks of (k–1)-subsets.
Retain a Counter-based multiplicity check (< length − 1) for robustness and to match existing doctest behavior.
Inline comments clarifying the rationale.
Why
Standard Apriori datasets (unique items per transaction) benefit from faster membership-only checks.
Keeping multiplicity check ensures correctness for edge cases.
File changed:
[apriori_algorithm.py]
Tests:
Doctests (module): 11 passed, 0 failed.
Reference:
Apriori algorithm: [https://en.wikipedia.org/wiki/Apriori_algorithm]
Fixes #12943

Checklist:
[x]I have read CONTRIBUTING.md.
[x]This pull request is all my own work -- I have not plagiarized.
[x]I know that pull requests will not be merged if they fail the automated tests.
[x] This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
[x]All new Python files are placed inside an existing directory. (No new files added.)
[x]All filenames are in all lowercase characters with no spaces or dashes.
[x]All functions and variable names follow Python naming conventions.
[x] All function parameters and return values are annotated with Python type hints.
[x] 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. (Not a new algorithm.)
[x] If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: “Fixes #12943”.

[algorithms-keeper]

Multiple Pull Request Detected

@kdt523, we are extremely excited that you want to submit multiple algorithms in this repository but we have a limit on how many pull request a user can keep open at a time. This is to make sure all maintainers and users focus on a limited number of pull requests at a time to maintain the quality of the code.

This pull request is being closed as the user already has an open pull request. Please focus on your previous pull request before opening another one. Thank you for your cooperation.

User opened pull requests (including this one): #13616, #13615, #13555, #13538