.NET: fix: Add session support for Handoff-hosted Agents

[lokitoth]

Motivation and Context

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.

Description

• Moves handoff message accumulate to shared state
• Adds bookmarking support to HandoffAgentExecutor to properly subselect only new messages when invoking the underlying agent
• Makes Handoff agent hosts use a single session per workflow session
• Closes .NET Workflows: RequestPort in Subworkflow (AsAgent) included in Parentworkflow (Handoff) never receives SendResponseAsync response #3766

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• [ ] Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

This PR updates the .NET handoff orchestration to use shared state + bookmarking and to keep a single AgentSession per workflow session, improving checkpointing behavior and ensuring only new messages are sent to underlying agents.

Changes:

• Move handoff message accumulation into shared orchestration state (HandoffSharedState + MultiPartyConversation).
• Add bookmarking support so each agent processes only new messages per invocation.
• Introduce message filtering extraction and expand/adjust unit tests for handoff orchestration behavior.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 10 comments.

Show a summary per file

File	Description
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/TestRunContext.cs	Adds a StateManager to test contexts and wires state operations to it.
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/SampleSmokeTest.cs	Updates Step12 expected-output computation to reflect bookmarking/session behavior.
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/Sample/12_HandOff_HostAsAgent.cs	Minor formatting adjustment in the Step12 sample.
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/HandoffAgentExecutorTests.cs	Initializes shared state for tests and adds a new HITL/subworkflow composition test.
dotnet/src/Microsoft.Agents.AI.Workflows/Specialized/MultiPartyConversation.cs	New shared conversation buffer with bookmarking support.
dotnet/src/Microsoft.Agents.AI.Workflows/Specialized/HandoffState.cs	Removes per-message payload from HandoffState (messages now live in shared state).
dotnet/src/Microsoft.Agents.AI.Workflows/Specialized/HandoffStartExecutor.cs	Initializes/updates shared orchestration state and emits bookmark-driven HandoffState.
dotnet/src/Microsoft.Agents.AI.Workflows/Specialized/HandoffMessagesFilter.cs	Extracts tool-call filtering logic into a dedicated type.
dotnet/src/Microsoft.Agents.AI.Workflows/Specialized/HandoffEndExecutor.cs	Reads from shared state and yields the accumulated conversation at workflow end.
dotnet/src/Microsoft.Agents.AI.Workflows/Specialized/HandoffAgentExecutor.cs	Adds agent-session use, bookmarking, and shared-state persistence for conversation messages.
dotnet/src/Microsoft.Agents.AI.Workflows/AIAgentsAbstractionsExtensions.cs	Broadens role-changing helper to accept IEnumerable<ChatMessage>.