🤖 Apply Copilot suggestions: Improve code examples

- Remove undefined function references (handleFlags, handleMetadata)
- Add proper error handling instead of silently discarding errors
- Keep external plugin example simple and focused on custom markers
- Ensure all code examples are self-contained and functional

Addresses Copilot PR review feedback

Author: nerdeveloper

docs/book/src/plugins/extending/custom-markers.md

@@ -156,10 +156,6 @@ func main() {
     switch pluginRequest.Command {
     case "init":
         response = handleInit(pluginRequest)
-    case "flags":
-        response = handleFlags(pluginRequest)
-    case "metadata":
-        response = handleMetadata(pluginRequest)
     default:
         response = external.PluginResponse{
             Command: pluginRequest.Command,
@@ -174,7 +170,14 @@ func main() {
 
 func handleInit(req *external.PluginRequest) external.PluginResponse {
     // Create Rust file with custom markers
-    marker, _ := markers.NewRustMarker("src/main.rs", "imports")
+    marker, err := markers.NewRustMarker("src/main.rs", "imports")
+    if err != nil {
+        return external.PluginResponse{
+            Command: "init",
+            Error:   true,
+            ErrorMsgs: []string{fmt.Sprintf("failed to create Rust marker: %v", err)},
+        }
+    }
 
     fileContent := fmt.Sprintf(`// Generated by Rust Plugin
 %s