.NET: Add Message Delivery Callback Overloads to Executor

[lokitoth]

Motivation and Context

Often it is desirable to create an Executor that runs aggregation logic on incoming messages, but that logic is not incremental, so it is important to know when the full set of messages has been collected.

Description

Add OnMessageDeliveryStarting and OnMessageDeliveryFinished overloads on Executor, enabling users to determine when the super step's message deliveries for the Executor are starting and when deliveries have completed.

Closes #2156

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 introduces executor lifecycle hooks to let workflow authors detect when an executor’s per-superstep message delivery phase begins and ends—supporting non-incremental aggregation scenarios (e.g., fan-in) in the .NET workflows in-process runner.

Changes:

• Added Executor.OnMessageDeliveryStartingAsync / Executor.OnMessageDeliveryFinishedAsync virtual hooks.
• Invoked the new delivery hooks from the in-process runner around per-executor message delivery.
• Added unit tests validating delivery hooks and existing checkpointing/restore events behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
dotnet/src/Microsoft.Agents.AI.Workflows/Executor.cs	Adds new virtual per-superstep message delivery start/finish hooks on Executor.
dotnet/src/Microsoft.Agents.AI.Workflows/InProc/InProcessRunner.cs	Calls the new hooks before/after delivering queued messages to an executor in a superstep.
dotnet/src/Microsoft.Agents.AI.Workflows/InProc/InProcessExecutionEnvironment.cs	Updates resume behavior (but currently introduces a bug in returned Run).
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/InProcessExecutorEventsTests.cs	New tests covering delivery hooks plus checkpointing/restore event behavior.