refactor: [BREAKING] Remove generic Workflow<T> (#1551)

Remove input type checking in favour of explicit `.DescribeProtocolAsync()` flow. Also removes `.AsAgentAsync()` as the validation happens at workflow run time. This makes it easier to use Workflows with DI without resorting to async-over-sync.

dotnet/samples/GettingStarted/Workflows/_Foundational/02_Streaming/Program.cs

@@ -28,7 +28,7 @@ public static class Program
         var workflow = builder.Build();
 
         // Execute the workflow in streaming mode
-        await using StreamingRun run = await InProcessExecution.StreamAsync(workflow, "Hello, World!");
+        await using StreamingRun run = await InProcessExecution.StreamAsync(workflow, input: "Hello, World!");
         await foreach (WorkflowEvent evt in run.WatchStreamAsync())
         {
             if (evt is ExecutorCompletedEvent executorCompleted)

dotnet/samples/GettingStarted/Workflows/_Foundational/05_MultiModelService/Program.cs

@@ -57,8 +57,7 @@ AIAgent reporter = new ChatClientAgent(anthropic,
     description: "Summarize the researcher's essay into a single paragraph, focusing only on the fact checker's confirmed facts.");
 
 // Build a sequential workflow: Researcher -> Fact-Checker -> Reporter
-AIAgent workflowAgent = await AgentWorkflowBuilder.BuildSequential(researcher, factChecker, reporter)
-                                                  .AsAgentAsync();
+AIAgent workflowAgent = AgentWorkflowBuilder.BuildSequential(researcher, factChecker, reporter).AsAgent();
 
 // Run the workflow, streaming the output as it arrives.
 string? lastAuthor = null;