Refactor CancelablePromise

[Pterjudin]

This PR improves code clarity and maintainability by:

• Extracting repeated state checks into a isFinalState() helper.
• Using ReadonlyArray for cancelHandlers for immutability.
• Encapsulating onCancel logic in a createOnCancel() method.
• Improving cancel() with better error handling and cleanup.
• Adding explicit type annotations for private fields.
  *Ensuring consistent use of semicolons and adding JSDoc comments.

NB:No changes to functionality; purely refactoring for readability and robustness.

[tiangolo]

This is all code in the automatically generated client, so it shouldn't really be modified by hand. 😅

So, I'll pass on this one, but thanks for the interest! ☕