* 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
* feat: Implement return-to-previous routing in handoff workflow
- Also obsoletes HandoffsWorkflowBuilder => HandoffWorkflowBuilder (no "s")
* refactor: Remove instance-shared current agent tracking in handoffs
Because the tracker was instance-shared between the start and end executors, it would be shared between all sessions, resulting in incorrect behaviour.
The corect way to do this is to keep the data in a shared executor scope, which is per-session.
* fix: Fix test logic for Handoff to correctly use checkpointing for multiturn
* refactor: remove unused internals
* feat: Execution Mode for sharing a workflow among concurrent runs
* feat: Update WorkflowHostAgent to support concurrent execution
* Also update AsAgent APIs to support injecting a CheckpointManager and an IWorkflowExecutionEnvironment
* fix: Make Read logic consistent in DeclarativeWorkflowContext
