gh-137928: Centralize size validation in multiprocessing.heap

[acabelloj]

gh-137928: Refactor to centralize size validation in multiprocessing.heap

Summary

This PR refactors the multiprocessing.heap module by centralizing size validation into a private static method (Heap._validate_size).
Previously, the same validation logic was duplicated in both Heap.malloc() and BufferWrapper.__init__().

Changes

• Introduced Heap._validate_size(size, _maxsize=sys.maxsize).
• Updated malloc() and BufferWrapper.__init__() to use this method.
• Bound sys.maxsize at definition time to avoid repeated global lookups.

Impact

• No behavior or API changes.
• Internal refactor only: reduces duplication, improves maintainability, and ensures consistent validation across the module.
• Existing tests already cover these code paths.

[python-cla-bot]

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