release 2.5.9

chore: bump to 2.5.9

• feat: add exponential backoff, jitter and conditional retry triggering
  BackoffStrategy enum (FIXED/EXPONENTIAL); EXPONENTIAL uses base*2^attempt capped at maxIntervalMs;
  jitter factor [0,1] to avoid thundering herd;
  retryStatusCodes / retryExceptionClasses to narrow trigger conditions on top of RetryRule;