feat(provider): add health check infrastructure

[agustif]

Summary

Adds optional health check infrastructure for providers to validate connection availability before SDK initialization.

Changes

• ✅ Add healthCheck config schema with url, timeout, and enabled options
• ✅ Implement checkProviderHealth function using AbortSignal.timeout
• ✅ Integrate health check into getSDK (lazy, non-blocking, cached)
• ✅ Add HealthCheckError for health check failures
• ✅ Update error formatting for health check errors
• ✅ Add health check result caching in provider state

Behavior

• Health checks are automatically enabled for local providers (127.0.0.1 or localhost)
• Health checks can be explicitly configured for any provider via config
• Health checks are lazy - only run when SDK is first requested
• Health checks are non-blocking - failures log warnings but don't prevent SDK creation
• Health check results are cached per provider to avoid repeated checks

Example Config

{
  "provider": {
    "lmstudio": {
      "healthCheck": {
        "url": "http://127.0.0.1:1234/v1/models",
        "timeout": 2000,
        "enabled": true
      }
    }
  }
}

Testing

• Health check passes when server is online
• Health check fails gracefully when server is offline
• Health check respects timeout
• Health check doesn't block provider initialization
• Health check results are cached
• Local providers automatically get health checks

Related

Part of LM Studio support improvements. See packages/opencode/docs/LM_STUDIO_DEEP_REVIEW.md for full context.