feat: Replace alertMixin usage with useAlert (#9793)

# Pull Request Template

## Description

This PR will replace the usage of `alertMixin` from the code base with
the `useAlert` composable.

Fixes
https://linear.app/chatwoot/issue/CW-3462/replace-alertmixin-usage-with-usealert

## Type of change

- [x] Breaking change (fix or feature that would cause existing
functionality not to work as expected)

## How Has This Been Tested?

Please refer this issue description

https://linear.app/chatwoot/issue/CW-3462/replace-alertmixin-usage-with-usealert


## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [ ] 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

---------

Co-authored-by: Sojan Jose <sojan@pepalo.com>

app/javascript/dashboard/mixins/specs/fileUploadMixin.spec.js

@@ -1,5 +1,6 @@
-import fileUploadMixin from 'dashboard/mixins/fileUploadMixin';
 import Vue from 'vue';
+import { useAlert } from 'dashboard/composables';
+import fileUploadMixin from 'dashboard/mixins/fileUploadMixin';
 
 vi.mock('shared/helpers/FileHelper', () => ({
   checkFileSizeLimit: vi.fn(),
@@ -11,6 +12,10 @@ vi.mock('activestorage', () => ({
   })),
 }));
 
+vi.mock('dashboard/composables', () => ({
+  useAlert: vi.fn(),
+}));
+
 describe('FileUploadMixin', () => {
   let vm;
 
@@ -53,9 +58,8 @@ describe('FileUploadMixin', () => {
 
     it('shows an alert if the file size exceeds the maximum limit', () => {
       const fakeFile = { size: 999999999 };
-      vm.showAlert = vi.fn();
       vm.onDirectFileUpload(fakeFile);
-      expect(vm.showAlert).toHaveBeenCalledWith(expect.any(String));
+      expect(useAlert).toHaveBeenCalledWith(expect.any(String));
     });
   });
 
@@ -67,9 +71,8 @@ describe('FileUploadMixin', () => {
 
     it('shows an alert if the file size exceeds the maximum limit', () => {
       const fakeFile = { size: 999999999 };
-      vm.showAlert = vi.fn();
       vm.onIndirectFileUpload(fakeFile);
-      expect(vm.showAlert).toHaveBeenCalledWith(expect.any(String));
+      expect(useAlert).toHaveBeenCalledWith(expect.any(String));
     });
   });
 });