[aleksUIX]

Summary

Closes #139.

Pass a seller base URL to every UnifiedClient, IABMCPClient, and A2AClient construction in the five affected examples. The examples default to http://localhost:8000 and accept SELLER_BASE_URL for another seller endpoint.

Validation

• Ruff passed on all five changed examples.
• Python compilation passed on all five changed examples.
• An AST check confirmed that all seven client constructions in those examples supply base_url.
• git diff --check passed.