make AssistantAgent and Handoff use BaseTool

[victordibia]

Why are these changes needed?

Make AssistantAgent and Handoff use BaseTool.
This ensures that they can be made declarative/serialized

Related issue number

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.08%. Comparing base (b6597fd) to head (42e6bee).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5193      +/-   ##
==========================================
- Coverage   70.08%   70.08%   -0.01%     
==========================================
  Files         179      179              
  Lines       11620    11619       -1     
==========================================
- Hits         8144     8143       -1     
  Misses       3476     3476              

Flag	Coverage Δ
unittests	70.08% <100.00%> (-0.01%)	⬇️

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[victordibia]

@jackgerrits @ekzhu ,

It seems we are hitting the covariant/contravariant issue with BaseToool similar to component config.

ArgsT = TypeVar("ArgsT", bound=BaseModel, contravariant=True)
ReturnT = TypeVar("ReturnT", bound=BaseModel, covariant=True)
StateT = TypeVar("StateT", bound=BaseModel)

Now that we have BaseTool[BaseModel, BaseModel] as a component, we get mypy errors when child classes pass types for ArgsT and ReturnT that are not specifically a BaseModel.

The fix will likely be to remove the contravariant=True but I am still thinking through side effects here (less flexibility for inheriting classes). Thoughts welcom.

Edit: Another (lighter) solution is to slightly relax the AssistantAgent tools type .. from List[BaseTool[BaseModel, BaseModel] to List[BaseTool[Any, Any] . It is already a very liberal type already (accepts CallableAny). Going with this!!

[ekzhu]

I think Any is fine.