Add support for system message in Claude Notebook

[levscaut]

Why are these changes needed?

Claude example does not yet support system message. This PR add support for system message of Anthropic API.

Related issue number

#2246

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 ✅

Project coverage is 38.17%. Comparing base (46bee1f) to head (04a2305).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2280   +/-   ##
=======================================
  Coverage   38.17%   38.17%           
=======================================
  Files          78       78           
  Lines        7812     7812           
  Branches     1669     1669           
=======================================
  Hits         2982     2982           
  Misses       4581     4581           
  Partials      249      249           

Flag	Coverage Δ
unittests	38.15% <ø> (ø)

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