Fix Go doc example: type-assert SessionEventData to AssistantMessageData

The Go SDK uses per-event-type data structs, so response.Data is a
SessionEventData interface. Access Content by type-asserting to
*copilot.AssistantMessageData.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: stephentoub

docs/setup/local-cli.md

@@ -99,7 +99,9 @@ func main() {
 
 	session, _ := client.CreateSession(ctx, &copilot.SessionConfig{Model: "gpt-4.1"})
 	response, _ := session.SendAndWait(ctx, copilot.MessageOptions{Prompt: "Hello!"})
-	fmt.Println(*response.Data.Content)
+	if d, ok := response.Data.(*copilot.AssistantMessageData); ok {
+		fmt.Println(d.Content)
+	}
 }
 ```
 <!-- /docs-validate: hidden -->
@@ -115,7 +117,9 @@ defer client.Stop()
 
 session, _ := client.CreateSession(ctx, &copilot.SessionConfig{Model: "gpt-4.1"})
 response, _ := session.SendAndWait(ctx, copilot.MessageOptions{Prompt: "Hello!"})
-fmt.Println(*response.Data.Content)
+if d, ok := response.Data.(*copilot.AssistantMessageData); ok {
+    fmt.Println(d.Content)
+}
 ```
 
 </details>