.NET: feat: Implement return-to-previous routing in handoff workflow

[lokitoth]

Motivation and Context

It is often desirable to return control to the user in a handoff orchestration session without losing the context of which Agent is active (and returning to the original starting agent).

Description

• Implement return-to-previous routing in handoff workflow
• Also obsoletes HandoffsWorkflowBuilder => HandoffWorkflowBuilder (no "s")
• Closes .NET: Feature Request: Add EnableReturnToPrevious() to HandoffsWorkflowBuilder in .NET SDK #2504

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

This PR implements return-to-previous routing in handoff workflows, allowing subsequent user turns to route directly back to the specialist agent that handled the previous turn, rather than always routing through the initial coordinator agent. The PR also deprecates HandoffsWorkflowBuilder in favor of HandoffWorkflowBuilder (without the 's').

Changes:

• Added EnableReturnToPrevious() method to configure return-to-previous routing behavior
• Introduced HandoffsCurrentAgentTracker to track the active agent across turns
• Modified HandoffState to include CurrentAgentId for routing decisions
• Deprecated HandoffsWorkflowBuilder and introduced HandoffWorkflowBuilder as the preferred name

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
HandoffsWorkflowBuilder.cs	Adds class hierarchy with obsolete HandoffsWorkflowBuilder and new HandoffWorkflowBuilder; implements EnableReturnToPrevious() method and dynamic routing logic
HandoffsCurrentAgentTracker.cs	New simple tracker class to maintain current agent ID across workflow turns
HandoffState.cs	Adds optional CurrentAgentId parameter to track active agent
HandoffsStartExecutor.cs	Updated to accept tracker and pass current agent ID to HandoffState
HandoffsEndExecutor.cs	Updated to accept tracker and update current agent ID at workflow completion
HandoffAgentExecutor.cs	Determines current agent ID based on handoff target and stores mapping
AgentWorkflowBuilderTests.cs	Comprehensive tests covering default behavior, enabled return-to-previous, and edge cases

Comments suppressed due to low confidence (1)

dotnet/src/Microsoft.Agents.AI.Workflows/HandoffsWorkflowBuilder.cs:39

• The constructor parameter name in the XML documentation incorrectly refers to HandoffsWorkflowBuilder (with 's'), but this is now in the base class HandoffWorkflowBuilderCore. The documentation should be updated to refer to the correct class name or remain generic.

    /// <summary>
    /// Initializes a new instance of the <see cref="HandoffsWorkflowBuilder"/> class with no handoff relationships.
    /// </summary>
    /// <param name="initialAgent">The first agent to be invoked (prior to any handoff).</param>
    internal HandoffsWorkflowBuilder(AIAgent initialAgent)