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>

spec/presenters/message_content_presenter_spec.rb

@@ -10,8 +10,8 @@ RSpec.describe MessageContentPresenter do
       let(:content_type) { 'text' }
       let(:content) { 'Regular message' }
 
-      it 'returns regular content' do
-        expect(presenter.outgoing_content).to eq('Regular message')
+      it 'returns content transformed for channel (HTML for WebWidget)' do
+        expect(presenter.outgoing_content).to eq("<p>Regular message</p>\n")
       end
     end
 
@@ -23,8 +23,8 @@ RSpec.describe MessageContentPresenter do
         allow(message.inbox).to receive(:web_widget?).and_return(true)
       end
 
-      it 'returns regular content without survey URL' do
-        expect(presenter.outgoing_content).to eq('Rate your experience')
+      it 'returns content without survey URL (HTML for WebWidget)' do
+        expect(presenter.outgoing_content).to eq("<p>Rate your experience</p>\n")
       end
     end
 
@@ -36,18 +36,20 @@ RSpec.describe MessageContentPresenter do
         allow(message.inbox).to receive(:web_widget?).and_return(false)
       end
 
-      it 'returns I18n default message when no CSAT config and dynamically generates survey URL' do
+      it 'returns I18n default message when no CSAT config and dynamically generates survey URL (HTML format)' do
         with_modified_env 'FRONTEND_URL' => 'https://app.chatwoot.com' do
           expected_url = "https://app.chatwoot.com/survey/responses/#{conversation.uuid}"
           expect(presenter.outgoing_content).to include(expected_url)
+          expect(presenter.outgoing_content).to include('<p>')
         end
       end
 
-      it 'returns CSAT config message when config exists and dynamically generates survey URL' do
+      it 'returns CSAT config message when config exists and dynamically generates survey URL (HTML format)' do
         with_modified_env 'FRONTEND_URL' => 'https://app.chatwoot.com' do
           allow(message.inbox).to receive(:csat_config).and_return({ 'message' => 'Custom CSAT message' })
           expected_url = "https://app.chatwoot.com/survey/responses/#{conversation.uuid}"
-          expect(presenter.outgoing_content).to eq("Custom CSAT message #{expected_url}")
+          expected_content = "<p>Custom CSAT message #{expected_url}</p>\n"
+          expect(presenter.outgoing_content).to eq(expected_content)
         end
       end
     end