improvement: fallback API providers for resilience

[satoshai-dev]

Summary

Support multiple API endpoints so tests don't fail when a single provider (e.g., Hiro) has an outage.

Problem

All API calls go to a single hardcoded URL per network (e.g., `api.hiro.so` for mainnet). If Hiro's API is down or rate-limited, all tests fail.

Proposed Solution

```typescript
network: {
name: 'mainnet',
apiUrls: [
'https://api.hiro.so',
'https://stacks-node-api.mainnet.stacks.co',
],
}
```

• Try the primary URL first
• On failure (timeout, 5xx, network error), retry with the next URL
• Configurable via `PlaystacksConfig`

Requirements

• Accept `apiUrls` array in network config (backwards-compatible, single URL still works)
• Implement retry-with-fallback in `api-client.ts`
• Respect request timeout (improvement: request timeouts for API calls #14)