Feature Parity: Add Cancellation Support to Python Package

[AryanSharma48]

Description

In PR #40 , native cancellation support was added to the TypeScript package (smooth-api-ts) allowing users to pass an AbortSignal to cancel requests and interrupt backoff delays immediately.

To maintain feature parity as defined in our Contributing guidelines, we need to implement equivalent cancellation behavior in the Python package (smooth-api-py).

Acceptance Criteria

• Implement a mechanism to cancel requests and interrupt backoff delays in Python. (The idiomatic approach is usually properly handling asyncio.CancelledError when the wrapping task is cancelled, or accepting an asyncio.Event as a cancellation token).
• Ensure that cancelling a request immediately breaks out of the retry loop and doesn't just trigger another retry.
• Add unit tests verifying cancellation during the backoff sleep phase and pre-aborted states.
• Update packages/smooth-api-py/README.md with usage examples.

Additional Context

• Reference TS Implementation PR: feat: Add Abort Controller #40
• Python does not have a 1:1 equivalent of AbortController. The implementation should feel native to Python asyncio patterns.