.NET: Fix race condition issue in FanInEdge while processing messages.

[peibekwe]

Motivation and Context

FanInEdgeState.ProcessMessage() is called concurrently from parallel executor tasks during RunSuperstepAsync (via Task.WhenAll) but uses non-thread-safe List.Add and HashSet.Remove. This race condition causes messages to be lost non-deterministically and can lead to premature workflow termination.

Description

Added a lock around the ProcessMessage implementation to serialize concurrent access. Using a lock approach here instead of concurrent collections because the operations within ProcessMessage (add message, remove from unseen, check count, swap messages, reset unseen) are interdependent and must execute atomically as a group. Making individual collections thread-safe would not prevent interleaving between these compound steps. The critical section is tiny and synchronous, so there shouldn't be a contention concern.

Fixes #3060

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 race condition in the .NET workflow FanInEdge implementation by serializing concurrent FanInEdgeState.ProcessMessage calls, preventing message loss and premature workflow termination when supersteps execute in parallel.

Changes:

• Added a lock in FanInEdgeState.ProcessMessage to make compound updates to pending messages and unseen-source tracking atomic.
• Added a unit test that exercises concurrent FanInEdgeRunner.ChaseEdgeAsync calls to validate correct batch release under contention.

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/Execution/FanInEdgeState.cs	Serializes ProcessMessage state mutations with a lock to eliminate concurrent List/HashSet races.
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/EdgeRunnerTests.cs	Adds a concurrency-focused test to ensure exactly one batch release containing all source messages per iteration.

[lokitoth]

Synced with Peter offline about the changes. Will switch to comment to avoid blocking. Hmm, apparently clearing status no longer works. Okay; I'll just keep an eye on this.