docs(sampling): clarify client-side sampling context

[MukundaKatta]

Summary

• explain that ctx.session.create_message(...) routes sampling through the connected client rather than exposing a direct server-side model handle
• document how to use model_preferences, include_context, and the client-side sampling callback together
• update the README callback example to use ClientRequestContext and sync the snippet source files with the clarified guidance

Testing

• not run (docs/snippets only)

Closes #1205

[MukundaKatta]

Closing out — pre-commit failures and I'm not pushing updates. Thanks.