[Documentation] Update tools.ipynb: use system messages in the tool_agent_caller_loop session

[zysoong]

…oop session

Why are these changes needed?

The _system_messages in the ToolUseAgent example is defined but never used in the session.

Related issue number

n.a.

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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 57.64%. Comparing base (8bd65c6) to head (12c1cfc).
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5068       +/-   ##
===========================================
- Coverage   72.95%   57.64%   -15.32%     
===========================================
  Files         116       75       -41     
  Lines        6800     6001      -799     
===========================================
- Hits         4961     3459     -1502     
- Misses       1839     2542      +703     

Flag	Coverage Δ
unittests	57.64% <ø> (-15.32%)	⬇️

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