Fix ZeroDivisionError in polynom_for_points by adding partial pivoting

[zain-cs]

Describe your change:

Fixes #14817

points_to_polynomial crashed with ZeroDivisionError for any input
containing x=0. The Gaussian elimination had no pivoting, so when
matrix[count][count] was zero it divided by zero.

Added partial pivoting before each elimination step — swaps in the row
with the largest absolute value in the current column, preventing
zero-division and improving numerical stability. Updated doctests to
reflect the corrected output.

• 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?

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 linear_algebra/polynom_for_points: ZeroDivisionError on valid points containing x=0 (Gaussian elimination without pivoting) #14817".