chore: Repalce message formatter mixin with useMessageFormatter [CW-3470] (#9986)

# Pull Request Template

## Description

Replaced the old messageFormatterMixin with a useMessageFormatter
composable

app/javascript/shared/composables/specs/useMessageFormatter.spec.js

@@ -0,0 +1,79 @@
+import { useMessageFormatter } from '../useMessageFormatter';
+
+describe('useMessageFormatter', () => {
+  let messageFormatter;
+
+  beforeEach(() => {
+    messageFormatter = useMessageFormatter();
+  });
+
+  describe('formatMessage', () => {
+    it('should format a regular message correctly', () => {
+      const message = 'This is a [test](https://example.com) message';
+      const result = messageFormatter.formatMessage(message, false, false);
+      expect(result).toContain('<a href="https://example.com"');
+      expect(result).toContain('class="link"');
+    });
+
+    it('should format a tweet correctly', () => {
+      const message = '@user #hashtag';
+      const result = messageFormatter.formatMessage(message, true, false);
+      expect(result).toContain('<a href="http://twitter.com/user"');
+      expect(result).toContain('<a href="https://twitter.com/hashtag/hashtag"');
+    });
+
+    it('should not format mentions and hashtags for private notes', () => {
+      const message = '@user #hashtag';
+      const result = messageFormatter.formatMessage(message, false, true);
+      expect(result).not.toContain('<a href="http://twitter.com/user"');
+      expect(result).not.toContain(
+        '<a href="https://twitter.com/hashtag/hashtag"'
+      );
+    });
+  });
+
+  describe('truncateMessage', () => {
+    it('should not truncate short messages', () => {
+      const message = 'Short message';
+      const result = messageFormatter.truncateMessage(message);
+      expect(result).toBe(message);
+    });
+
+    it('should truncate long messages', () => {
+      const message = 'A'.repeat(150);
+      const result = messageFormatter.truncateMessage(message);
+      expect(result.length).toBe(100);
+      expect(result.endsWith('...')).toBe(true);
+    });
+  });
+
+  describe('highlightContent', () => {
+    it('should highlight search term in content', () => {
+      const content = 'This is a test message';
+      const searchTerm = 'test';
+      const highlightClass = 'highlight';
+      const result = messageFormatter.highlightContent(
+        content,
+        searchTerm,
+        highlightClass
+      );
+      expect(result.trim()).toBe(
+        'This is a <span class="highlight">test</span> message'
+      );
+    });
+
+    it('should handle special characters in search term', () => {
+      const content = 'This (message) contains [special] characters';
+      const searchTerm = '(message)';
+      const highlightClass = 'highlight';
+      const result = messageFormatter.highlightContent(
+        content,
+        searchTerm,
+        highlightClass
+      );
+      expect(result.trim()).toBe(
+        'This <span class="highlight">(message)</span> contains [special] characters'
+      );
+    });
+  });
+});