* 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
* Update OpenTelemetryAgent to latest spec
It was stale. Rather than try to keep it up-to-date manually, I've changed it to piggy back on OpenTelemetryChatClient, so that it inherits everything OpenTelemetryChatClient does and then augments it just with agent-specific tags.
* Address feedback / merge