chore: Replace messageMixing with useMessage composable [CW-3475] (#9942)

# Pull Request Template

## Description

Replaces the messageMixin with the new useMessage composable

Fixes
https://linear.app/chatwoot/issue/CW-3475/rewrite-messagemixin-mixin-to-a-composable

## Type of change

Please delete options that are not relevant.

- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality not to work as expected)
- [ ] This change requires a documentation update

app/javascript/widget/mixins/messageMixin.js

@@ -1,13 +0,0 @@
-export default {
-  computed: {
-    messageContentAttributes() {
-      const { content_attributes: attribute = {} } = this.message;
-      return attribute;
-    },
-    hasAttachments() {
-      return !!(
-        this.message.attachments && this.message.attachments.length > 0
-      );
-    },
-  },
-};

app/javascript/widget/mixins/specs/messageMixin.spec.js

@@ -1,37 +0,0 @@
-import { shallowMount } from '@vue/test-utils';
-import messageMixin from '../messageMixin';
-import messages from './messageFixture';
-
-describe('messageMixin', () => {
-  let Component = {
-    render() {},
-    title: 'TestComponent',
-    mixins: [messageMixin],
-  };
-
-  it('deleted messages', async () => {
-    const wrapper = shallowMount(Component, {
-      data() {
-        return {
-          message: messages.deletedMessage,
-        };
-      },
-    });
-    expect(wrapper.vm.messageContentAttributes).toEqual({
-      deleted: true,
-    });
-    expect(wrapper.vm.hasAttachments).toBe(false);
-  });
-  it('non-deleted messages', async () => {
-    const wrapper = shallowMount(Component, {
-      data() {
-        return {
-          message: messages.nonDeletedMessage,
-        };
-      },
-    });
-    expect(wrapper.vm.deletedMessage).toBe(undefined);
-    expect(wrapper.vm.messageContentAttributes).toEqual({});
-    expect(wrapper.vm.hasAttachments).toBe(true);
-  });
-});