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/Orchestration/HandoffOrchestration_With_StructuredInput.cs

@@ -2,10 +2,8 @@
 
 using System.Text.Json.Serialization;
 using Microsoft.Agents.Orchestration;
-using Microsoft.Agents.Orchestration.Handoff;
 using Microsoft.Extensions.AI;
 using Microsoft.Extensions.AI.Agents;
-using Microsoft.Extensions.AI.Agents.Runtime.InProcess;
 
 namespace Orchestration;
 
@@ -49,10 +47,7 @@ public class HandoffOrchestration_With_StructuredInput(ITestOutputHelper output)
         HandoffOrchestration<GithubIssue, string> orchestration =
             new(OrchestrationHandoffs
                     .StartWith(triageAgent)
-                    .Add(triageAgent, dotnetAgent, pythonAgent),
-                triageAgent,
-                pythonAgent,
-                dotnetAgent)
+                    .Add(triageAgent, dotnetAgent, pythonAgent))
             {
                 LoggerFactory = this.LoggerFactory,
                 ResponseCallback = monitor.ResponseCallback,
@@ -83,18 +78,11 @@ public class HandoffOrchestration_With_StructuredInput(ITestOutputHelper output)
                 Labels = []
             };
 
-        // Start the runtime
-        await using InProcessRuntime runtime = new();
-        await runtime.StartAsync();
-
         // Run the orchestration
         Console.WriteLine($"\n# INPUT:\n{input.Id}: {input.Title}\n");
-        OrchestrationResult<string> result = await orchestration.InvokeAsync(input, runtime);
-        string text = await result.GetValueAsync(TimeSpan.FromSeconds(ResultTimeoutInSeconds));
-        Console.WriteLine($"\n# RESULT: {text}");
+        OrchestrationResult<string> result = await orchestration.InvokeAsync(input);
+        Console.WriteLine($"\n# RESULT: {await result}");
         Console.WriteLine($"\n# LABELS: {string.Join(",", githubPlugin.Labels["12345"])}\n");
-
-        await runtime.RunUntilIdleAsync();
     }
 
     private sealed class GithubIssue