When MultiPartyConversation gets saved during checkpointing, the data for the chat history is not persisted, resulting in failures to deserialize after. The fix is to make the history visible to the source generated serialization code.
* fix: Add session support for Handoff-hosted Agents
In order to better support using `Workflows` hosted as `AIAgents` inside of Handoff workflows, we need to make proper use of AgentSession. This causes potential issues around checkpointing and making sure that we properly compute only the new incoming messages for each agent invocation.
* fix: AgentSession checkpointing using AIAgent's Serialize/Deserialize methods
We cannot rely on implicit serialization through `HandoffHostState` because we are missing type information.
* fix: Thread safety issue in `MultiPartyConversation.AllMessages`
* fix: Enable unwrapping of FunctionResultContent when ExternalRequest was wrapped into FunctionCallContent
* feat: Refactor Handoff Orchestration and add HITL support
* Change HandoffAgentExecutor to use factory-based instantiation
* Extract shared request collection logic in AIAgentUnservicedRequestsCollector
* Refactor HandoffAgentExecutor to use the "ContinueTurn" pattern as in AIAgentHostExecutor
* fix: Remove '$' from exception strings