Round 2 of cleanup for agent runtime and orchestrations (#164)

- Moved InProcessRuntime type into abstractions package and deleted InProcess package.
- Moved several members of IAgentRuntime to be extension methods instead, e.g. multiple GetActorAsync overloads.
- Added synchronous RegisterMessageHandler overloads and used them to avoid unnecessary async usage at call sites.
- Removed unnecessary surface area from InProcessRuntime, e.g. StopAsync, RunUntilIdleAsync, etc.
- Fixed spin loop in InProcessRuntime that would consume an entire core for the duration of the orchestration's operation.
- Removed a bunch of allocation from InProcessRuntime.
- Made a runtime optional for orchestrations, defaulting to using a temporary InProcessRuntime if none is provided.
- Removed custom delegate types from orchestrations.
- Consolidated namespaces.
- Used records to simplify message classes.
- Tweaked naming on AgentActor to make purpose of protected methods more clear.
- Removed invocation in AgentActor.InvokeAsync of empty update / isFinal parameter.
- Changed OrchestrationHandoffs to avoid needing to pass in agents duplicatively.
- Made various extension methods, such as those on OrchestrationHandoffsExtensions, into instance methods.

dotnet/samples/GettingStarted/AgentSample.cs

@@ -159,11 +159,8 @@ public class AgentSample(ITestOutputHelper output) : BaseSample(output)
 
     private async Task AzureAIAgentsPersistentAgentCleanUpAsync(ChatClientAgent agent, AgentThread? thread, CancellationToken cancellationToken)
     {
-        var persistentAgentsClient = agent.ChatClient.GetService<PersistentAgentsClient>();
-        if (persistentAgentsClient is null)
-        {
+        var persistentAgentsClient = agent.ChatClient.GetService<PersistentAgentsClient>() ??
             throw new InvalidOperationException("The provided chat client is not a Persistent Agents Chat Client");
-        }
 
         await persistentAgentsClient.Administration.DeleteAgentAsync(agent.Id, cancellationToken);