Document CancellationToken propagation through nested tools

[Whxuan0701]

Summary

Document how CancellationToken should pass from an agent message handler through a nested tool or workbench call.

Current implementation facts

AutoGen exposes cancellation-aware APIs across core and extension packages.

Problem

Top-level cancellation can appear successful while a nested browser, network, or computation tool continues if the token is not forwarded.

Scope

• Add a deterministic cancellable nested operation.
• Show the cancellation result.
• Identify boundaries that must forward the token.

Non-goals

• Do not redesign cancellation APIs.
• Do not add provider-specific behavior.

Acceptance criteria

• No external credentials are required.
• The nested operation observes cancellation.
• Public types are named precisely.

I searched AutoGen issues and PRs for CancellationToken propagation, nested tools, and cancellation documentation before filing this proposal.