fix: type k-means 3D plotting and enable parameter-already-assigned

[gcomneno]

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?

plt.axes(projection="3d") creates an Axes3D, but its static return type
is the base 2D Axes. Consequently, ty interprets the third centroid
coordinate as s and reports a collision with the explicit s=100.

Cast the created axis to Axes3D and annotate all three parameters of
plot_kmeans as NumPy arrays. With no remaining
parameter-already-assigned diagnostics repository-wide, remove that
rule's ignore entry from pyproject.toml.

Part of #15187.

This follows the approach discussed in the closed PR #15463 and includes
the parameter annotations requested in its review:
#15463 (comment)

Validation

• All 22 module doctests passed on standard CPython 3.15.0rc2.
• The targeted ty check passed with parameter-already-assigned enabled.
• The repository-wide ty check reported zero diagnostics for that rule;
  59 other diagnostics remain. The full ty baseline is not clean.
• An Agg-backend comparison against the original implementation confirmed
  identical coordinates, marker sizes, face colors, marker paths, labels,
  and rendered pixels for the tested input.
• Applicable pre-commit hooks and git diff --check passed.
• The full repository test suite was not run.

AI assistance

ChatGPT helped investigate the diagnostic and prepare the patch and
verification commands. I reviewed the diff and ran the reported checks locally.

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.

No new files or algorithms are added. Existing unannotated functions and
functions without doctests remain outside this change; the modified
plot_kmeans signature is fully annotated. The tracking issue remains open.