fix: CodeExecutorAgent prompt misuse

[Dormiveglia-elf]

Why are these changes needed?

Summary of Change:
The instruction regarding code block format ("Python code should be provided in python code blocks, and sh shell scripts should be provided in sh code blocks for execution") will be moved from DEFAULT_AGENT_DESCRIPTION to DEFAULT_SYSTEM_MESSAGE.

Problem Solved:
Ensure that the model_client receives the correct instructions for generating properly formatted code blocks. Previously, the instruction was only included in the agent's description and not passed to the model_client, leading to potential issues in code generation. By moving it to DEFAULT_SYSTEM_MESSAGE, the model_client will now accurately format code blocks, improving the reliability of code generation.

Related issue number

Closes #6558

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md 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 79.52%. Comparing base (fffa61f) to head (63c827d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6559      +/-   ##
==========================================
+ Coverage   75.40%   79.52%   +4.11%     
==========================================
  Files         331      225     -106     
  Lines       19779    16661    -3118     
  Branches      406        0     -406     
==========================================
- Hits        14914    13249    -1665     
+ Misses       4592     3412    -1180     
+ Partials      273        0     -273     

Flag	Coverage Δ
unittests	79.52% <100.00%> (+4.11%)	⬆️

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