Allow InProcessRuntime subscriptions to be processed concurrently

[stephentoub]

No description provided.

[Copilot]

Pull Request Overview

Allows in-process runtime subscriptions to run in parallel instead of sequentially by collecting processing tasks and awaiting them together.

• Switches from catching and aggregating exceptions to spawning tasks and using Task.WhenAll
• Introduces a static local function ProcessSubscriptionAsync for each subscription
• Changes the topic-match check to add matching subscriptions to the task list

Comments suppressed due to low confidence (3)

dotnet/src/Microsoft.Extensions.AI.Agents.Runtime.Abstractions/InProcessRuntime.cs:265

• [nitpick] The variable name tasks is generic and may be ambiguous; consider renaming it to subscriptionTasks to clarify that it holds subscription processing tasks.

            List<Task>? tasks = null;

dotnet/src/Microsoft.Extensions.AI.Agents.Runtime.Abstractions/InProcessRuntime.cs:297

• This change introduces parallel execution of subscription handlers. Please add or update tests to verify behavior when multiple subscriptions run concurrently, including cases with successful, faulted, and cancelled tasks.

                await Task.WhenAll(tasks).ConfigureAwait(false);

dotnet/src/Microsoft.Extensions.AI.Agents.Runtime.Abstractions/InProcessRuntime.cs:271

• Launching all subscription handlers as concurrent tasks can introduce race conditions if the underlying ISubscriptionDefinition implementations or actor interactions are not thread-safe; ensure those components can handle parallel execution or serialize access where necessary.

                    (tasks ??= []).Add(ProcessSubscriptionAsync(message, subscription.Value, topic, cancellationToken));

[crickman]

Beautiful

[stephentoub]

How important is it that we throw an AggregateException with all the exceptions? I neglected to do so, such that right now tests are failing because only the first exception is propagating. I can fix it to be an aggregate if that's important...? Otherwise I'll fix the tests.