Run the examples in CI with mocked network calls

[TheCoder30ec4]

Problem

The examples aren't tested, so an API change can break them without anyone noticing (for example when Router became keyword-only).

Proposal

Add tests/test_examples.py:

• import examples/basic.py and examples/agent.py
• patch model_router.router.fetch_catalog, model_router.jev.request_json and the chat call
• run each main() with a fake key set in the environment

Done when

• Both examples run offline in CI
• Tests fail if an example uses an API that doesn't exist