fix: use averaged ranks for ties in Spearman, guard n<2 - #14889

Fixes #14887

Changes made:

• Fixed assign_ranks() to use averaged ranks for tied values instead of sequential ranks (e.g. [1, 2, 2, 4] now correctly gives [1.0, 2.5, 2.5, 4.0] instead of [1.0, 2.0, 3.0, 4.0]).
• Added ValueError guard for n<1 to prevent ZeroDivisionError if a single data point is passed.
• Updated existing doctest for [5, 1, 2, 9, 5] from 0.6 to 0.4, the old value was itself incorrect, produced by the broken sequential ranking.
• Updated return type annotation on assign_ranks() from list[int] to list[float] since averaged ranks like (2.5) are floats.
• Added new doctests covering tied values and the n=1 edge case.

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 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.