Fix: Correct cancellation token usage in UserProxyAgent docstring

[slow-loop]

Why are these changes needed?

The example code in UserProxyAgent's docstring has a bug where timeout handling doesn't work. The code creates a new CancellationToken instead of using the shared token that gets cancelled by the timeout callback, making the timeout ineffective.

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation 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.

[jackgerrits]

Thanks!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.20%. Comparing base (16ceded) to head (8afabac).
Report is 738 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4874      +/-   ##
==========================================
- Coverage   68.21%   68.20%   -0.02%     
==========================================
  Files         158      158              
  Lines        9960     9960              
==========================================
- Hits         6794     6793       -1     
- Misses       3166     3167       +1     

Flag	Coverage Δ
unittests	68.20% <ø> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.