Helpers/retry: interrupt backoff sleep on context cancellation

What does this MR do?

Interrupts the retry backoff sleep when the provided context is cancelled, allowing retry loops to exit promptly instead of waiting for the full backoff duration.

Why was this MR needed?

The existing retry behavior continues sleeping even after context cancellation. This can unnecessarily delay shutdown paths and cancellation handling, especially in failure or teardown scenarios where responsiveness is important.

What's the best way to test this MR?

Run the unit tests for the retry helper and the full test suite:

go test ./helpers/retry

go test ./...

What are the relevant issue numbers?

N/A (no existing issue linked)