feat: Cleanup serialization of content

[baxen]

This better aligns with client expectations

{"result":[{"Text":{"text":"example"}}],"toolCallId":"call_HZM0sraJtEL0znZd4xaSbckj"}

becomes

{"result":[{"text":"example","type":"text"}],"toolCallId":"call_AFJtX3C1mJdzRov6qt72iVZB"}