.NET: fix: WorkflowAsAgent Sample

[lokitoth]

When we started introducing the different execution modes to solidify the concurrency model, we broke the WorkflowAsAgent Sample, which was not updated for either of IResettableExecutor or Cross-Run Shareable.

The fix is to make public and switch to using the ChatForwardingExecutor in place of a self-defined ConcurrentStartExecutor, and make the ConcurrentAggregationExecutor implement IResettableExecutor

Description

• Fixes the Workflow in WorkflowAsAgent sample to be ChatProtocol and resettable (closes .NET: Cannot reuse Workflow with shared Executor instances that do not implement IResettableExecutor #1613)
• Also makes ChatForwardingExecutor public

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 refactors the ChatForwardingExecutor from an internal specialized component to a public reusable API, and updates the WorkflowAsAnAgent sample to use it instead of a custom implementation.

Key changes:

• Made ChatForwardingExecutor public with comprehensive XML documentation
• Moved it from Microsoft.Agents.AI.Workflows.Specialized to Microsoft.Agents.AI.Workflows namespace
• Replaced custom ConcurrentStartExecutor in sample with ChatForwardingExecutor
• Added IResettableExecutor implementation to ConcurrentAggregationExecutor for proper cleanup

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
dotnet/src/Microsoft.Agents.AI.Workflows/ChatForwardingExecutor.cs	Promoted from internal to public, added comprehensive documentation, and marked ConfigureRoutes as sealed override
dotnet/samples/GettingStarted/Workflows/Agents/WorkflowAsAnAgent/WorkflowFactory.cs	Replaced custom ConcurrentStartExecutor with ChatForwardingExecutor and added reset capability to ConcurrentAggregationExecutor

Comments suppressed due to low confidence (2)

dotnet/src/Microsoft.Agents.AI.Workflows/ChatForwardingExecutor.cs:20

• The sealed modifier on an override method is redundant since the containing class is already sealed. Remove sealed from the method declaration.
  dotnet/src/Microsoft.Agents.AI.Workflows/ChatForwardingExecutor.cs:12
• The newly public ChatForwardingExecutor class lacks unit test coverage. Consider adding tests to verify message forwarding behavior for all supported message types (string, ChatMessage, List, TurnToken).

[lokitoth]

Added one more fix to update for a change in how message aggregation happens in AIAgentHostExecutor post #1439

[lokitoth]

Pending #1612