.NET: Fix to emit WorkflowStartedEvent during workflow execution

[peibekwe]

Description

Fix bug to emit WorkflowStartedEvent during workflow execution.
Fixes #3789

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

Fixes a .NET workflows bug where WorkflowStartedEvent was not emitted to streaming consumers during workflow execution (issue #3789), and adds regression tests to verify the event is produced.

Changes:

• Emit WorkflowStartedEvent into the streaming event stream in off-thread execution.
• Emit WorkflowStartedEvent into the streaming event stream in lockstep execution.
• Add unit tests validating WorkflowStartedEvent emission and relative ordering vs SuperStepStartedEvent.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/AgentEventsTests.cs	Adds regression tests for WorkflowStartedEvent emission and ordering.
dotnet/src/Microsoft.Agents.AI.Workflows/Execution/StreamingRunEventStream.cs	Writes WorkflowStartedEvent into the channel at the start of each run-loop iteration.
dotnet/src/Microsoft.Agents.AI.Workflows/Execution/LockstepRunEventStream.cs	Enqueues WorkflowStartedEvent at the start of lockstep streaming.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

[Copilot]

In lockstep mode, WorkflowStartedEvent is only enqueued into the local eventSink here, but the sink is drained/yielded only after a RunSuperStepAsync completes. If the first RunSuperStepAsync throws (or the iterator exits before draining), consumers may never observe WorkflowStartedEvent. To make the event reliable and consistent with StreamingRunEventStream, emit/yield the WorkflowStartedEvent before entering the first superstep execution (and ensure it is only emitted once per run cycle).

Suggested change

	// Emit WorkflowStartedEvent to the event stream for consumers
	eventSink.Enqueue(new WorkflowStartedEvent());
	// Emit WorkflowStartedEvent directly so it is observed even if the first superstep fails
	yield return new WorkflowStartedEvent();