Fix quick_select crashing on non-positive rank in median_of_medians.py

[Saadanjum0]

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?

quick_select in searches/median_of_medians.py only guarded against
target > len(arr). A rank of 0 or a negative rank slipped past that
check and was passed into median_of_medians() on a shrinking slice of
arr until it indexed an empty list, raising an unhandled IndexError
instead of returning -1 the way other invalid input already does.

Fixed by rejecting target <= 0 alongside the existing upper-bound
check, and added two doctests (target = 0, target = -1) that
reproduce the crash before the fix and pass after it.

Checklist

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