fix: withConfig creates independent HTTP client from merged config (Closes #405)

[Zhiyilang074811]

Summary

Fixes \LilySdk.withConfig\ to create a new HTTP client from the merged config instead of reusing the parent instance's transport. This ensures multi-tenant scenarios route requests to the correct baseUrl and credentials.

Changes

• src/sdk.ts: Removed the duplicate/broken \withConfig\ implementation that passed \ his.httpClient\ to the new instance. Kept only the correct implementation that creates a fresh SDK instance without sharing the parent's transport.
• tests/sdk-withconfig-routing.test.ts: Added integration tests verifying:
  • Derived instance routes requests through overridden baseUrl and credentials
  • Original instance continues routing to its own baseUrl/credentials

Acceptance Criteria

• \sdk.withConfig({ baseUrl: 'https://tenant.example.com', apiKey: 'tenant-key' }).agents.list()\ sends requests to tenant.example.com with tenant-key
• Overriding only apiKey/authToken changes auth headers on the wire
• Original instance still routes to its own baseUrl/credentials
• Explicitly injected custom HttpClient behavior preserved (no httpClient passed = new one created from config)

Closes #405

[Zhiyilang074811]

Hi! Friendly ping — open for a couple of days, all checks passing. Ready for review when convenient. Thanks! 🙏

[Zhiyilang074811]

Hi! Friendly ping — open for ~2 days, all checks passing. Ready for review when convenient. Thanks! 🙏