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Create a cancelable promise wrapper

MIDDLE

Implement a wrapper that exposes cancel() and prevents handlers from running after cancellation, without leaking memory.

Example:

Input: value = 42
Output: 42

Write a clean implementation and validate it with the provided test cases.

Examples:

Input 1:{"value":1}
Output 1:1
Input 2:{"value":42}
Output 2:42

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