chore: Replace plain editor with advanced editor (#13071)

# Pull Request Template

## Description

This PR reverts the plain text editor back to the **advanced editor**,
which was previously removed in
[https://github.com/chatwoot/chatwoot/pull/13058](https://github.com/chatwoot/chatwoot/pull/13058).
All channels now use the **ProseMirror editor**, with formatting applied
based on each channel’s configuration.

This PR also fixes issues where **new lines were not properly preserved
during Markdown serialization**, for both:

* `Enter or CMD/Ctrl+enter` (new paragraph)
* `Shift+Enter` (`hard_break`)

Additionally, it resolves related **[Sentry
issue](https://chatwoot-p3.sentry.io/issues/?environment=production&project=4507182691975168&query=is%3Aunresolved%20markdown&referrer=issue-list&statsPeriod=7d)**.

With these changes:

* Line breaks and spacing are now preserved correctly when saving canned
responses.
* When editing a canned response, the content retains the exact spacing
and formatting as saved in editor.
* Canned responses are now correctly converted to plain text where
required and displayed consistently in the canned response list.

### https://github.com/chatwoot/prosemirror-schema/pull/38

---

## Type of change

- [x] New feature (non-breaking change which adds functionality)

## How Has This Been Tested?



## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

---------

Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>

app/javascript/dashboard/components/widgets/mentions/MentionBox.vue

@@ -1,6 +1,7 @@
 <script setup>
 import { ref, watch, computed, nextTick } from 'vue';
 import { useKeyboardNavigableList } from 'dashboard/composables/useKeyboardNavigableList';
+import { useMessageFormatter } from 'shared/composables/useMessageFormatter';
 
 const props = defineProps({
   items: {
@@ -15,6 +16,8 @@ const props = defineProps({
 
 const emit = defineEmits(['mentionSelect']);
 
+const { getPlainText } = useMessageFormatter();
+
 const mentionsListContainerRef = ref(null);
 const selectedIndex = ref(0);
 
@@ -94,7 +97,7 @@ const variableKey = (item = {}) => {
                 'text-n-slate-12': index === selectedIndex,
               }"
             >
-              {{ item.description }}
+              {{ getPlainText(item.description) }}
             </p>
             <p
               class="max-w-full min-w-0 mb-0 overflow-hidden text-xs text-n-slate-11 group-hover:text-n-slate-12 text-ellipsis whitespace-nowrap"