* 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
* Upgrade to .NET 10
- Require .NET 10 SDK
- Include net10.0 assets in all assemblies
- Move net9.0-only targets to net10.0
- Update LangVersion to latest
- Remove complicated distinctions between debug target TFMs and release target TFMs
- Remove unnecessary package dependencies when built into netcoreapp
- Clean up some ifdefs
- Clean up some analyzer warnings
* Fix CI