Add maximum matching in trees using dynamic programming

[stupidprogrammer4]

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?

Adds graphs/maximum_matching_tree.py to find a maximum cardinality matching in an unweighted tree using dynamic programming. It returns the selected edges in O(n) time and O(n) auxiliary space, using iterative traversal to support long paths without recursion limits.

For example, the path 0–1–2–3 produces [(0, 1), (2, 3)]. The function accepts vertices numbered from zero, handles empty and single-vertex trees, and raises ValueError for invalid tree inputs. The new implementation includes type hints, an algorithm reference, and doctests covering valid and invalid inputs.

Validation

• All 12 doctest examples in the new file passed; its doctest also passed under pytest on Python 3.14.2.
• An additional local verification script compared matching sizes against exhaustive edge-subset search for all 1,441 labelled trees with 2 through 6 vertices. It also checked that returned edges belong to the input and have no shared endpoints.
• Additional checks passed for a 20,000-vertex path and a 20,000-vertex star.
• All applicable hooks in pre-commit run --all-files passed in a temporary checkout containing the changes, with no automatic file modifications.

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". Not applicable: this PR does not claim to resolve an existing issue.