[flutter] Update pubspec.lock files and generated plugin registrant

- Updated spine-flutter/example/pubspec.lock to reflect version 4.3.1
- Updated spine-flutter/test/pubspec.lock to reflect version 4.3.1
- Updated GeneratedPluginRegistrant.swift with Flutter-generated formatting

These files were out of sync with the main pubspec.yaml version, causing
CI format checks to fail when flutter pub get regenerated them.

spine-flutter/example/macos/Flutter/GeneratedPluginRegistrant.swift

@@ -5,5 +5,6 @@
 import FlutterMacOS
 import Foundation
 
+
 func RegisterGeneratedPlugins(registry: FlutterPluginRegistry) {
 }

spine-flutter/example/pubspec.lock

@@ -169,7 +169,7 @@ packages:
       path: ".."
       relative: true
     source: path
-    version: "4.3.0"
+    version: "4.3.1"
   string_scanner:
     dependency: transitive
     description:

spine-flutter/test/pubspec.lock

@@ -329,7 +329,7 @@ packages:
       path: ".."
       relative: true
     source: path
-    version: "4.3.0"
+    version: "4.3.1"
   stack_trace:
     dependency: transitive
     description: