Resilient Fetch Wrapper with Jittered Exponential Backoff

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interface RetryOptions {
  maxRetries?: number;
  baseDelayMs?: number;
  retryOn?: number[];
}

export async function fetchWithRetry(url: string, options: RequestInit = {}, retryConfig: RetryOptions = {}): Promise<Response> {
  const { maxRetries = 3, baseDelayMs = 300, retryOn = [408, 429, 500, 502, 503, 504] } = retryConfig;

  for (let attempt = 0; attempt <= maxRetries; attempt++) {
    try {
      const res = await fetch(url, options);
      if (!retryOn.includes(res.status) || attempt === maxRetries) {
        return res;
      }
    } catch (err) {
      if (attempt === maxRetries) throw err;
    }

    const jitter = Math.random() * 0.3 + 0.85; // ±15% variation
    const delay = Math.min(baseDelayMs * Math.pow(2, attempt) * jitter, 10000);
    await new Promise((r) => setTimeout(r, delay));
  }
  throw new Error("Maximum retry attempts exceeded");
}