Configures Req's built-in retry mechanism with Banking-Circle-appropriate policy: capped exponential backoff with full jitter, honoring a server-provided Retry-After when present (notably on 429s from the auth and payment endpoints).

Rather than hand-rolling request/response steps against Req's internals, this module builds the :retry, :retry_delay, and :max_retries options that Req's own retry step already understands — this keeps us aligned with upstream Req behavior across version bumps instead of reimplementing it.

Retry policy

• GET/HEAD requests are always eligible (safe by HTTP semantics).
• Other methods are only eligible if the request carries an idempotency-key header — Banking Circle only guarantees idempotency on endpoints that support that header today (Direct Debit Collections at time of writing). A bare POST without one is never auto-retried, to avoid duplicate payment submission.
• Eligible statuses: 429, 500, 502, 503, 504.
• Eligible transport errors: timeout, closed, econnrefused, econnreset.