.NET Workflows - Declarative update for AzureAgentProvider usage (#907)

* Updated

* Update dotnet/src/Microsoft.Agents.Workflows.Declarative/Extensions/IWorkflowContextExtensions.cs

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Constant / override

* Namespace

* Protect against race

* Simplify collection usage

* Rollback name resolution

* One more rollback

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

workflow-samples/Marketing.yaml

@@ -1,5 +1,8 @@
 #
-# This workflow demonstrates a single agent interaction based on user input.
+# This workflow demonstrates sequential agent interaction to develop product marketing copy.
+#
+# Example input: 
+# An eco-friendly stainless steel water bottle that keeps drinks cold for 24 hours.
 #
 # Any Foundry Agent may be used to provide the response.
 # See: ./setup/QuestionAgent.yaml
@@ -11,13 +14,19 @@ trigger:
   id: workflow_demo
   actions:
 
+    - kind: AddConversationMessage
+      id: add_input_message
+      conversationId: =System.ConversationId
+      content:
+        - type: Text
+          value: {System.LastMessage.Text}   
+
     - kind: InvokeAzureAgent
       id: invoke_analyst
       conversationId: =System.ConversationId
       agent:
         name: =Env.FOUNDRY_AGENT_ANSWER
       input:
-        messages: =[UserMessage(System.LastMessageText)]
         additionalInstructions: |-
           You are a marketing analyst. Given a product description, identify:
           - Key features