Function defaultRetryPolicy

The default retry policy (decision D3). An attempt is retried iff all hold:

1. attempts so far < maxRetries + 1 (enforced by the retry loop bounds)
2. error kind ∈ {network, timeout, retryable_status}
3. method is idempotent (GET HEAD OPTIONS PUT DELETE) OR ctx.idempotent === true OR the request has an Idempotency-Key header
4. user retryWhen (consulted separately via shouldRetry) returns true
5. the request is not cancelled / past its deadline (handled elsewhere)