chore: Repalce message formatter mixin with useMessageFormatter [CW-3470] (#9986)

# Pull Request Template

## Description

Replaced the old messageFormatterMixin with a useMessageFormatter
composable

app/javascript/dashboard/components/widgets/conversation/MessagePreview.vue

@@ -1,11 +1,10 @@
 <script>
 import { MESSAGE_TYPE } from 'widget/helpers/constants';
-import messageFormatterMixin from 'shared/mixins/messageFormatterMixin';
+import { useMessageFormatter } from 'shared/composables/useMessageFormatter';
 import { ATTACHMENT_ICONS } from 'shared/constants/messages';
 
 export default {
   name: 'MessagePreview',
-  mixins: [messageFormatterMixin],
   props: {
     message: {
       type: Object,
@@ -20,6 +19,12 @@ export default {
       default: '',
     },
   },
+  setup() {
+    const { getPlainText } = useMessageFormatter();
+    return {
+      getPlainText,
+    };
+  },
   computed: {
     messageByAgent() {
       const { message_type: messageType } = this.message;