feat: Add support for HTML emails in outgoing messages (#12662)

This PR adds sending custom HTML content in outgoing email messages
through Chatwoot's Email channels, while maintaining backward
compatibility with existing markdown rendering.

###  API Usage

**Endpoint:** `POST
/api/v1/accounts/{account_id}/conversations/{conversation_id}/messages`

```json
  {
   "content": "Fallback text content",
   "email_html_content": "<div><h1>Welcome!</h1><p>This is <strong>custom HTML</strong></p></div>"
  }
```

---------

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

spec/mailers/conversation_reply_mailer_spec.rb

@@ -335,6 +335,118 @@ RSpec.describe ConversationReplyMailer do
           expect(mail.body.encoded).not_to match(%r{<a [^>]*>avatar\.png</a>})
         end
       end
+
+      context 'with custom email content' do
+        it 'uses custom HTML content when available and creates multipart email' do
+          message_with_custom_content = create(:message,
+                                               conversation: conversation,
+                                               account: account,
+                                               message_type: 'outgoing',
+                                               content: 'Regular message content',
+                                               content_attributes: {
+                                                 email: {
+                                                   html_content: {
+                                                     reply: '<p>Custom <strong>HTML</strong> content for email</p>'
+                                                   },
+                                                   text_content: {
+                                                     reply: 'Custom text content for email'
+                                                   }
+                                                 }
+                                               })
+
+          mail = described_class.email_reply(message_with_custom_content).deliver_now
+
+          # Check HTML part contains custom HTML content
+          html_part = mail.html_part || mail
+          expect(html_part.body.encoded).to include('<p>Custom <strong>HTML</strong> content for email</p>')
+          expect(html_part.body.encoded).not_to include('Regular message content')
+
+          # Check text part contains custom text content
+          text_part = mail.text_part
+          if text_part
+            expect(text_part.body.encoded).to include('Custom text content for email')
+            expect(text_part.body.encoded).not_to include('Regular message content')
+          end
+        end
+
+        it 'falls back to markdown rendering when custom HTML content is not available' do
+          message_without_custom_content = create(:message,
+                                                  conversation: conversation,
+                                                  account: account,
+                                                  message_type: 'outgoing',
+                                                  content: 'Regular **markdown** content')
+
+          mail = described_class.email_reply(message_without_custom_content).deliver_now
+
+          html_part = mail.html_part || mail
+          expect(html_part.body.encoded).to include('<strong>markdown</strong>')
+          expect(html_part.body.encoded).to include('Regular')
+        end
+
+        it 'handles empty custom HTML content gracefully' do
+          message_with_empty_content = create(:message,
+                                              conversation: conversation,
+                                              account: account,
+                                              message_type: 'outgoing',
+                                              content: 'Regular **markdown** content',
+                                              content_attributes: {
+                                                email: {
+                                                  html_content: {
+                                                    reply: ''
+                                                  }
+                                                }
+                                              })
+
+          mail = described_class.email_reply(message_with_empty_content).deliver_now
+
+          html_part = mail.html_part || mail
+          expect(html_part.body.encoded).to include('<strong>markdown</strong>')
+          expect(html_part.body.encoded).to include('Regular')
+        end
+
+        it 'handles nil custom HTML content gracefully' do
+          message_with_nil_content = create(:message,
+                                            conversation: conversation,
+                                            account: account,
+                                            message_type: 'outgoing',
+                                            content: 'Regular **markdown** content',
+                                            content_attributes: {
+                                              email: {
+                                                html_content: {
+                                                  reply: nil
+                                                }
+                                              }
+                                            })
+
+          mail = described_class.email_reply(message_with_nil_content).deliver_now
+
+          expect(mail.body.encoded).to include('<strong>markdown</strong>')
+          expect(mail.body.encoded).to include('Regular')
+        end
+
+        it 'uses custom text content in text part when only text is provided' do
+          message_with_text_only = create(:message,
+                                          conversation: conversation,
+                                          account: account,
+                                          message_type: 'outgoing',
+                                          content: 'Regular message content',
+                                          content_attributes: {
+                                            email: {
+                                              text_content: {
+                                                reply: 'Custom text content only'
+                                              }
+                                            }
+                                          })
+
+          mail = described_class.email_reply(message_with_text_only).deliver_now
+
+          text_part = mail.text_part
+          if text_part
+            expect(text_part.body.encoded).to include('Custom text content only')
+            expect(text_part.body.encoded).not_to include('Regular message content')
+          end
+        end
+      end
     end
 
     context 'when smtp enabled for email channel' do