Preserve the radix tree root when deleting a branch

[dhairyajangir]

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?

Preserve the empty root node when deleting one of its last two branches. For example:

root = RadixNode()
root.insert_many(["apple", "banana"])
root.delete("apple")
root.find("banana")  # previously False; now True

Deletion previously compressed the root into its remaining child, although lookup starts from an empty sentinel root. Only nodes with a nonempty prefix are now eligible for this compression. Non-root compression, prefix-word deletion, and complete deletion followed by reinsertion remain supported.

Existing doctests and unittest cases are unchanged as requested by the contribution guidelines. Validation on free-threaded CPython 3.15.0rc2:

• 8 existing doctests and 2 existing unittest cases pass.
• External targeted and seeded set-oracle checks: baseline 12 failing scenarios; fixed 14 passing scenarios, including 58,622 membership comparisons over a structured corpus with prefix words.
• pre-commit run --all-files --show-diff-on-failure passes all applicable configured hooks.

AI assistance was used to prepare this change. A separate agent review independently checked 100 deletion orders, non-root compression, deletion/reinsertion, and the existing tests.

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.
• 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.
• 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 new file or algorithm is added, and no issue is claimed. The existing doctests pass; this change adds no tests or functions to the algorithm file.

[algorithms-keeper]

Multiple Pull Request Detected

@dhairyajangir, 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): #15487, #15486, #15485, #15484, #15483

[dhairyajangir]

Thanks for reviewing and merging #15483, #15484 and #15485. Those earlier PRs are now resolved, and I have no open PRs in this repository.

This PR and #15487 were automatically closed for exceeding the simultaneous-PR limit. Would you be willing to reopen this radix-root fix for review now? I can keep the heap fix queued and focus on this one first. The existing regression evidence is in the description; I can add focused doctests or a benchmark if you would like them.

I have left both PRs closed pending your guidance.