Document CancellationToken propagation through nested tools

[nicolasdmolina]

Why are these changes needed?

Top-level cancellation can look successful while a nested tool keeps running if a new CancellationToken is created at send_message or StaticWorkbench.call_tool. This documents the current forwarding contract and adds a credential-free nested example plus tests.

Callers must pass the same token instance through message handlers (ctx.cancellation_token), Workbench.call_tool, nested BaseTool.run_json, and in-flight I/O via link_future. Omitting the token creates a separate cancellation boundary. Linked cancellation raises asyncio.CancelledError; StaticWorkbench.call_tool does not convert that into a ToolResult error.

Related issue number

Closes #8091

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

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