.NET: BREAKING: Unify AgentResponse[Update] events as WorkflowOutputEvents

[Jacob Alber (lokitoth)]

Motivation and Context

The streaming AgentResponseUpdate and full AgentResponse events were added before the WorkflowOutputEvent mechanism was formalized, leading to a pair of vestigial events that cause confusion for users.

Reconciling the two will make it clearer how to listen to Workflow outputs, though we special-case it to continue avoiding the output filtering logic, via WithOutputFrom().

Description

• Python-parity: Breaking: Renames WorkflowOutputEvent.SourceId to WorkflowOutputEvent.ExecutorId
• Adds [Obsolete] SourceId forwarding to ExecutorId on WorkflowOutputEvent
• Changes AgentResponseUpdateEvent and AgentResponseEvent to derive from WorkflowOutputEvent to unify handling
• Fixes.NET : Agents in Workflow. WorkflowOutputEvent not triggered #2938

BREAKING CHANGES:

• Rename WorkflowOutputEvent.SourceId to WorkflowOutputEvent.ExecutorId

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 unifies agent response events under the WorkflowOutputEvent hierarchy to address confusion about how to listen to workflow outputs from agents. The change makes AgentResponseUpdateEvent and AgentResponseEvent derive from WorkflowOutputEvent instead of ExecutorEvent, and renames WorkflowOutputEvent.SourceId to ExecutorId for Python parity.

Changes:

• Renamed WorkflowOutputEvent.SourceId to ExecutorId with obsolete forwarding property for backward compatibility
• Changed AgentResponseUpdateEvent and AgentResponseEvent to inherit from WorkflowOutputEvent instead of ExecutorEvent
• Updated executors to use YieldOutputAsync() instead of directly calling AddEventAsync() for agent events, ensuring consistent special-case handling

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
WorkflowOutputEvent.cs	Changed class from sealed to non-sealed, renamed property SourceId to ExecutorId with obsolete forwarding, made constructor public, added JSON derived type attributes
AgentResponseUpdateEvent.cs	Changed base class from ExecutorEvent to WorkflowOutputEvent
AgentResponseEvent.cs	Changed base class from ExecutorEvent to WorkflowOutputEvent
AIAgentHostExecutor.cs	Updated to call YieldOutputAsync() instead of AddEventAsync() for agent events
HandoffAgentExecutor.cs	Updated to call YieldOutputAsync() instead of AddEventAsync() for agent events
InProcessRunnerContext.cs	Added special-case handling for AgentResponse and AgentResponseUpdate to create their specific event types
TestWorkflowContext.cs	Added special-case handling consistent with InProcessRunnerContext
TestRunContext.cs	Added special-case handling consistent with InProcessRunnerContext
AgentEventsTests.cs	New test file verifying the inheritance changes and regression test for issue #2938
Sample test files	Updated references from SourceId to ExecutorId