feat: Standardize rich editor across all channels (#12600)

# Pull Request Template

## Description

This PR includes,

1. **Channel-specific formatting and menu options** for the rich reply
editor.
2. **Removal of the plain reply editor** and full **standardization** on
the rich reply editor across all channels.
3. **Fix for multiple canned responses insertion:**
* **Before:** The plain editor only allowed inserting canned responses
at the beginning of a message, making it impossible to combine multiple
canned responses in a single reply. This caused inconsistent behavior
across the app.
* **Solution:** Replaced the plain reply editor with the rich
(ProseMirror) editor to ensure a unified experience. Agents can now
insert multiple canned responses at any cursor position.
4. **Floating editor menu** for the reply box to improve accessibility
and overall user experience.
5. **New Strikethrough formatting option** added to the editor menu.

---

**Editor repo PR**:
https://github.com/chatwoot/prosemirror-schema/pull/36

Fixes https://github.com/chatwoot/chatwoot/issues/12517,
[CW-5924](https://linear.app/chatwoot/issue/CW-5924/standardize-the-editor),
[CW-5679](https://linear.app/chatwoot/issue/CW-5679/allow-inserting-multiple-canned-responses-in-a-single-message)

## Type of change

- [x] Bug fix (non-breaking change which fixes an issue)

## How Has This Been Tested?

### Screenshot
**Dark**
<img width="850" height="345" alt="image"
src="https://github.com/user-attachments/assets/47748e6c-380f-44a3-9e3b-c27e0c830bd0"
/>

**Light**
<img width="850" height="345" alt="image"
src="https://github.com/user-attachments/assets/6746cf32-bf63-4280-a5bd-bbd42c3cbe84"
/>


## 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>
Co-authored-by: Pranav <pranav@chatwoot.com>
Co-authored-by: Vinay Keerthi <11478411+stonecharioteer@users.noreply.github.com>

app/javascript/dashboard/helper/specs/editorContentHelper.spec.js

@@ -1,15 +1,11 @@
 // Moved from editorHelper.spec.js to editorContentHelper.spec.js
 // the mock of chatwoot/prosemirror-schema is getting conflicted with other specs
 import { getContentNode } from '../editorHelper';
-import {
-  MessageMarkdownTransformer,
-  messageSchema,
-} from '@chatwoot/prosemirror-schema';
+import { MessageMarkdownTransformer } from '@chatwoot/prosemirror-schema';
 import { replaceVariablesInMessage } from '@chatwoot/utils';
 
 vi.mock('@chatwoot/prosemirror-schema', () => ({
   MessageMarkdownTransformer: vi.fn(),
-  messageSchema: {},
 }));
 
 vi.mock('@chatwoot/utils', () => ({
@@ -62,12 +58,18 @@ describe('getContentNode', () => {
       const to = 10;
       const updatedMessage = 'Hello John';
 
-      replaceVariablesInMessage.mockReturnValue(updatedMessage);
-      MessageMarkdownTransformer.mockImplementation(() => ({
-        parse: vi.fn().mockReturnValue({ textContent: updatedMessage }),
-      }));
+      // Mock the node that will be returned by parse
+      const mockNode = { textContent: updatedMessage };
 
-      const { node } = getContentNode(
+      replaceVariablesInMessage.mockReturnValue(updatedMessage);
+
+      // Mock MessageMarkdownTransformer instance with parse method
+      const mockTransformer = {
+        parse: vi.fn().mockReturnValue(mockNode),
+      };
+      MessageMarkdownTransformer.mockImplementation(() => mockTransformer);
+
+      const result = getContentNode(
         editorView,
         'cannedResponse',
         content,
@@ -79,8 +81,15 @@ describe('getContentNode', () => {
         message: content,
         variables,
       });
-      expect(MessageMarkdownTransformer).toHaveBeenCalledWith(messageSchema);
-      expect(node.textContent).toBe(updatedMessage);
+      expect(MessageMarkdownTransformer).toHaveBeenCalledWith(
+        editorView.state.schema
+      );
+      expect(mockTransformer.parse).toHaveBeenCalledWith(updatedMessage);
+      expect(result.node).toBe(mockNode);
+      expect(result.node.textContent).toBe(updatedMessage);
+      // When textContent matches updatedMessage, from should remain unchanged
+      expect(result.from).toBe(from);
+      expect(result.to).toBe(to);
     });
   });