Automatic retries with exponential backoff for 429/5xx

[Ashutosh0x]

Problem

Transient 429/500/503 responses currently surface straight to the caller. Every mature payment SDK (Stripe, Adyen, Square) retries these safely.

Proposal

• Retry idempotent requests (GET, and POSTs carrying an Idempotency-Key — see Support idempotency keys on all mutating requests #1) up to maxRetries (default 2) with exponential backoff + jitter.
• Respect Retry-After when present. Never retry 4xx other than 429.

Acceptance criteria

• Configurable maxRetries in the Furlpay constructor
• Backoff with jitter, Retry-After respected
• Tests with a mocked fetch: retry-then-succeed, exhaust-then-throw, no-retry-on-400