chore: Improve translation service with HTML and plain text support (#11305)

# Pull Request Template

## Description

This PR changes to translation to properly handle different content
types during translation.

### Changes

1. **Email translation with HTML support**
   - Properly detects and preserves HTML content from emails
   - Sets `mime_type` to 'text/html' when HTML content is present

2. **Email translation with plain text support**
   - Falls back to email text content when HTML is not available
- Sets `mime_type` to 'text/plain' when HTML is not available and
content type includes 'text/plain'

3. **Plain message with plain text support (Non email channels)**
   - Sets `mime_type` to 'text/plain' for non-email channels
- Fixes an issue where Markdown formatting was being lost due to
incorrect `mime_type`
   
**Note**: Translation for very long emails is not currently supported.

Fixes
https://linear.app/chatwoot/issue/CW-4244/translate-button-doesnt-work-in-email-channels

## Type of change

- [x] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)

## How Has This Been Tested?

**Loom video**

https://www.loom.com/share/8f8428ed2cfe415ea5cb6c547c070f00?sid=eab9fa11-05f8-4838-9181-334bee1023c4


## 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

lib/integrations/google_translate/processor_service.rb

@@ -1,15 +1,19 @@
 require 'google/cloud/translate/v3'
+
 class Integrations::GoogleTranslate::ProcessorService
   pattr_initialize [:message!, :target_language!]
 
   def perform
-    return if message.content.blank?
     return if hook.blank?
 
+    content = translation_content
+    return if content.blank?
+
     response = client.translate_text(
-      contents: [message.content],
+      contents: [content],
       target_language_code: target_language,
-      parent: "projects/#{hook.settings['project_id']}"
+      parent: "projects/#{hook.settings['project_id']}",
+      mime_type: mime_type
     )
 
     return if response.translations.first.blank?
@@ -19,6 +23,43 @@ class Integrations::GoogleTranslate::ProcessorService
 
   private
 
+  def email_channel?
+    message&.inbox&.email?
+  end
+
+  def email_content
+    @email_content ||= {
+      html: message.content_attributes.dig('email', 'html_content', 'full'),
+      text: message.content_attributes.dig('email', 'text_content', 'full'),
+      content_type: message.content_attributes.dig('email', 'content_type')
+    }
+  end
+
+  def html_content_available?
+    email_content[:html].present?
+  end
+
+  def plain_text_content_available?
+    email_content[:content_type]&.include?('text/plain') &&
+      email_content[:text].present?
+  end
+
+  def translation_content
+    return message.content unless email_channel?
+    return email_content[:html] if html_content_available?
+    return email_content[:text] if plain_text_content_available?
+
+    message.content
+  end
+
+  def mime_type
+    if email_channel? && html_content_available?
+      'text/html'
+    else
+      'text/plain'
+    end
+  end
+
   def hook
     @hook ||= message.account.hooks.find_by(app_id: 'google_translate')
   end