Reconcile divergent retry defaults and backoff formulas

[OmarAlJarrah]

Problem

Retryable-status sets disagree — RetryUtils includes 408 while RetrySettings.DEFAULT_RETRYABLE_STATUSES drops it — and there are two backoff formulas.

Proposed change

Make BackoffCalculator (deadline-aware + symmetric jitter) the single implementation; pick one stance on 408 and apply it everywhere. Keep both step families (they integrate at legitimately different points).

Acceptance

• Single backoff implementation
• Consistent retryable-status policy
• Tests

Dependencies

• Related Add an async retry step at the RETRY stage #31 — the async retry step reuses the unified backoff/defaults

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Priority: medium · Effort: medium