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

app/javascript/dashboard/composables/spec/useTranslations.spec.js

@@ -0,0 +1,39 @@
+import { ref } from 'vue';
+import { useTranslations } from '../useTranslations';
+
+describe('useTranslations', () => {
+  it('returns false and null when contentAttributes is null', () => {
+    const contentAttributes = ref(null);
+    const { hasTranslations, translationContent } =
+      useTranslations(contentAttributes);
+    expect(hasTranslations.value).toBe(false);
+    expect(translationContent.value).toBeNull();
+  });
+
+  it('returns false and null when translations are missing', () => {
+    const contentAttributes = ref({});
+    const { hasTranslations, translationContent } =
+      useTranslations(contentAttributes);
+    expect(hasTranslations.value).toBe(false);
+    expect(translationContent.value).toBeNull();
+  });
+
+  it('returns false and null when translations is an empty object', () => {
+    const contentAttributes = ref({ translations: {} });
+    const { hasTranslations, translationContent } =
+      useTranslations(contentAttributes);
+    expect(hasTranslations.value).toBe(false);
+    expect(translationContent.value).toBeNull();
+  });
+
+  it('returns true and correct translation content when translations exist', () => {
+    const contentAttributes = ref({
+      translations: { en: 'Hello' },
+    });
+    const { hasTranslations, translationContent } =
+      useTranslations(contentAttributes);
+    expect(hasTranslations.value).toBe(true);
+    // Should return the first translation (en: 'Hello')
+    expect(translationContent.value).toBe('Hello');
+  });
+});