Improve prefix_sum.py (#12560)

* Update prefix_sum.py

Index Validation for get_sum

Raises ValueError if start or end is out of range or start > end.
Handles cases where the array is empty.
✅ Empty Array Support

If an empty array is passed, get_sum raises an appropriate error instead of failing unexpectedly.
✅ Optimized contains_sum Initialization

Initializes sums with {0} for efficient subarray sum checking.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Update prefix_sum.py

* Update prefix_sum.py

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Update prefix_sum.py

* Update prefix_sum.py

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Maxim Smolskiy <mithridatus@mail.ru>