Fix tree_sort dropping duplicate values

[Darkslayer3324j]

Describe your change

• 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.

tree_sort silently drops repeated values, so it is not a sort of its input:

>>> tree_sort([1, 1])
(1,)
>>> tree_sort([3, 1, 3, 2, 1])
(1, 2, 3)      # expected (1, 1, 2, 3, 3)

Node.insert only handles val < self.val and val > self.val; a value equal to the node is ignored. The fix sends equal values to the right subtree so every element is kept. Doctests for repeated values are added to the docstring. I also compared it with sorted() on 18,000 random lists (lengths 0-59, values -4..4): 0 mismatches after the change.

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 open issue for this.)

I left "all my own work" unticked on purpose: this change was written with AI assistance (Claude Code, as AGENTS.md invites). I found the problem by comparing every function in sorts/ against sorted() on random lists, reproduced it, and ran the doctests, ruff check and ruff format --check on the changed file.