chore: import upstream snapshot with attribution

showcase/integrations/ms-agent-dotnet/agent/MultimodalAgent.cs

@@ -0,0 +1,60 @@
+using Microsoft.Agents.AI;
+using Microsoft.Extensions.AI;
+using OpenAI;
+
+// ============================================================================
+// Multimodal Agent
+// ============================================================================
+//
+// Vision-capable .NET agent for the Multimodal Attachments demo cell.
+//
+// Design mirrors the LangGraph reference
+// (showcase/integrations/langgraph-python/src/agents/multimodal_agent.py):
+//  - Use a vision-capable chat model (gpt-4o / gpt-4o-mini) so images are
+//    consumed natively by the model via OpenAI's image content parts.
+//  - No tools are registered — the model handles image/PDF analysis directly.
+//  - PDF handling: Microsoft.Extensions.AI passes document/data content parts
+//    through as DataContent, and modern OpenAI chat models accept PDF input
+//    directly. We therefore avoid bundling a PDF extractor (like pypdf on the
+//    Python side) and defer to the model's native document handling. If a PDF
+//    cannot be read, the model will tell the user — matching the "[Attached
+//    document: PDF could not be read.]" graceful degradation in Python.
+//
+// Wire format: `MultimodalEndpoint` parses the modern
+// `{ type: "image" | "document", source: {...} }` content parts CopilotChat
+// emits and forwards them as DataContent parts the chat client can pass to
+// the OpenAI image/file adapters unchanged. The dedicated endpoint exists
+// because the current Microsoft AG-UI ASP.NET adapter rejects content arrays
+// before an AIAgent can see them.
+//
+// Mount point: `/multimodal` (see Program.cs). The Next.js runtime's
+// `src/app/api/copilotkit-multimodal/route.ts` proxies to this endpoint via
+// AG-UI over HTTP.
+// ============================================================================
+
+internal static class MultimodalAgentFactory
+{
+    internal const string SystemPrompt =
+        "You are a helpful assistant. The user may attach images or documents " +
+        "(PDFs). When they do, analyze the attachment carefully and answer the " +
+        "user's question. If no attachment is present, answer the text question " +
+        "normally. Keep responses concise (1-3 sentences) unless asked to go deep.";
+
+    public static AIAgent Create(OpenAIClient openAiClient)
+    {
+        ArgumentNullException.ThrowIfNull(openAiClient);
+
+        // gpt-4o-mini supports vision natively. Matches the rest of the
+        // dotnet showcase (which uses gpt-4o-mini for every cell) so we don't
+        // introduce a new model id just for this cell. The LangGraph
+        // reference uses gpt-4o for slightly higher image-reasoning quality;
+        // gpt-4o-mini is cheaper and still vision-capable.
+        var chatClient = openAiClient.GetChatClient("gpt-4o-mini").AsIChatClient();
+
+        return new ChatClientAgent(
+            chatClient,
+            name: "MultimodalAgent",
+            description: SystemPrompt,
+            tools: []);
+    }
+}