fix: accept assistant with output_model when delegating tasks to a team

[nlenepveu]

Issue

When delegating a task to an assistant, the function call is encapsulated in a Message object. However, if the assistant utilizes an output_model, the validation for the Message object fails because the content attribute expects a string, but the output_model does not meet that requirement.

Fix

To resolve this, the output_model can be serialized into a JSON string before constructing the Message object, ensuring it passes the validation.