add long context handling notebook

[sonichi]

Why are these changes needed?

• Add notebook example
• Remove unused import
• Bump version to 0.2.13

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.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8089058) 37.74% compared to head (e5e6a57) 49.58%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1618       +/-   ##
===========================================
+ Coverage   37.74%   49.58%   +11.84%     
===========================================
  Files          50       50               
  Lines        5686     5683        -3     
  Branches     1289     1398      +109     
===========================================
+ Hits         2146     2818      +672     
+ Misses       3372     2640      -732     
- Partials      168      225       +57     

Flag	Coverage Δ
unittests	49.55% <100.00%> (+11.80%)	⬆️

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.

[sonichi]

@thinkall could you take a look at https://github.com/microsoft/autogen/actions/runs/7857025041/job/21440430989?pr=1618

Update: the error is gone after reunning.