Fix: Handle nested objects in array items for JSON schema conversion

[kkutrowski]

• Modified _extract_field_type to recursively process object schemas in arrays
• Added test case for array items with defined object properties

Why are these changes needed?

The changes are needed so the agents can see entire tools input schema properly.

Related issue number

Closes #6991

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.

[kkutrowski]

@microsoft-github-policy-service agree company="PFEIFER & LANGEN BUSINESS SOLUTIONS"

[ekzhu]

Generally speaking, it is a good idea to avoid using complex data types for tool arguments. They tend to degrade model performance. While the fix here is valid, we don't recommend passing in complex data types for tools.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.96%. Comparing base (17d3aef) to head (fe3d308).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6993   +/-   ##
=======================================
  Coverage   80.95%   80.96%           
=======================================
  Files         238      238           
  Lines       18271    18279    +8     
=======================================
+ Hits        14791    14799    +8     
  Misses       3480     3480           

Flag	Coverage Δ
unittests	80.96% <100.00%> (+<0.01%)	⬆️

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