Add retry with exponential backoff to HttpClient

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.

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

Locate the TypeScript HttpClient implementation and its HttpClientOptions definition, then trace how requests, errors, and AbortSignal cancellation are currently handled. Add focused coverage for retryable statuses, GET versus POST behavior, backoff limits, Retry-After, jitter, and cancellation; done means the documented defaults and opt-in behavior work without changing non-retryable requests.