Add jittered retry backoff controls

[RitwijParmar]

Summary

• default gateway retries to randomized exponential backoff so transient 5xx/429 bursts do not retry on a fixed synchronized schedule
• add retry config knobs for min_timeout, max_timeout, factor, and randomize, wired through validation, request context, and retryRequest
• document the bounded backoff config in the automatic retries cookbook

Why

Issue #1404 reports retry-induced p95/p99 tail amplification during upstream turbulence. The previous retryRequest path passed randomize: false to async-retry, which can make many retrying requests fire again on the same deterministic cadence. This gives operators a small, backwards-compatible way to jitter and bound retry delays while preserving the existing attempts/status-code behavior.

Validation

• npx jest tests/unit/src/handlers/retryHandler.test.ts --runInBand
• npx prettier --check cookbook/getting-started/automatic-retries-on-failures.md src/handlers/retryHandler.ts src/handlers/handlerUtils.ts src/handlers/services/requestContext.ts src/middlewares/requestValidator/schema/config.ts src/types/requestBody.ts tests/unit/src/handlers/retryHandler.test.ts
• git diff --check
• npm run build (passes; existing repo warnings about external deps/circular deps and pre-existing provider type warnings still appear)

Closes #1404