Add code samples for the "streaming API responses" page (#699)

Merging code sample only PR

Author: justinyoo

docs/guides/streaming-responses/responses/enable_streaming.cs

@@ -0,0 +1,29 @@
+// SAMPLE: Generate streaming responses through Responses API
+// PAGE: https://platform.openai.com/docs/guides/streaming-responses#enable-streaming
+// GUIDANCE: Instructions to run this code: https://aka.ms/oai/net/start
+#pragma warning disable OPENAI001
+
+#:package System.Linq.Async@6.*
+#:package OpenAI@2.*
+#:property PublishAot=false
+
+using OpenAI.Responses;
+
+string key = Environment.GetEnvironmentVariable("OPENAI_API_KEY")!;
+OpenAIResponseClient client = new(model: "gpt-5", apiKey: key);
+
+var responses = client.CreateResponseStreamingAsync([
+    ResponseItem.CreateUserMessageItem([
+        ResponseContentPart.CreateInputTextPart(
+            "Say 'double bubble bath' ten times fast."
+        )
+    ])
+]);
+
+await foreach (var response in responses)
+{
+    if (response is StreamingResponseOutputTextDeltaUpdate delta)
+    {
+        Console.Write(delta.Delta);
+    }
+}