Add a credential-free Python approval example matching the JavaScript refund demo

[aadilghani1]

The JavaScript examples/refund.mjs lets a visitor test the approval branch without an account or model key. Python users need the same starting point.

Scope: one Python example under examples/ and a short command in the Python README. Reuse the published Python adapter and a real LangGraph graph; simulate API responses and the refund only. Do not add a model provider or dependencies solely for the demo.

Acceptance:

• A clean Python environment can run the documented command with no credentials.
• The approval node routes to the simulated refund only for an explicit yes.
• Assertions prove no and unanswered execute zero refunds, while yes executes once.
• Output clearly says simulation; no network calls, push notifications, or real refunds occur.
• Include the Python/LangGraph versions and command output used to verify it.

Comment before starting so we can coordinate. Public PRs are welcome; see CONTRIBUTING.md for how reviewed patches are applied upstream, synced, and credited.