Add retry with exponential backoff to HttpClient

[rbren]

Problem

HttpClient has no retry logic. Transient failures (429 rate limit, 502/503/504 server errors, network blips) immediately throw. For a client library that manages long-running agent conversations, transient failures are expected and should be handled gracefully.

Proposed Fix

Add configurable retry with exponential backoff:

interface HttpClientOptions {
  baseUrl: string;
  apiKey?: string;
  timeout?: number;
  retry?: {
    maxRetries?: number;        // default: 3
    baseDelay?: number;         // default: 1000ms
    maxDelay?: number;          // default: 30000ms
    retryableStatuses?: number[]; // default: [429, 502, 503, 504]
  };
}

• Retry only idempotent requests (GET) by default, with opt-in for POST
• Use exponential backoff with jitter: delay = min(baseDelay * 2^attempt + random_jitter, maxDelay)
• Respect Retry-After header from 429 responses
• Support AbortSignal to cancel retries

Impact

Medium — improves reliability for production deployments where transient failures are common.

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This issue was created by an AI agent (OpenHands) on behalf of Robert Brennan.