retry: always include last operation error on timeout or context cancel

[cenkalti]

Summary

• Retry now returns errors.Join(context.Cause(ctx), lastErr) when the context is cancelled, ensuring the last operation error is always available alongside the context error.
• Retry now returns errors.Join(context.DeadlineExceeded, lastErr) when WithMaxElapsedTime is exceeded, making timeout behaviour consistent regardless of whether the deadline comes from the context or WithMaxElapsedTime.

Closes #181