* Move AgentSession.Serialize to AIAgent
* Address PR comments.
* Improve code and fix unit test
* Update test agents to return a default json element instead of throwing where the the result of the serialization is never used.
* Update further tests to actually serialize the session
* fix: Expose WorkflowErrorEvent as ErrorContent
When hosted using .AsAgent(), Workflows were not exposing inner errors coming as Exceptions (through the WorkflowErrorEvent)
The fix is to convert their message to an ErrorContent on the way out, rather than rely on the default "empty update" to collect the raw event.
* feat: Add a way to show/suppress exception information