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/fileUploadMixin.js

@@ -1,3 +1,4 @@
+import { useAlert } from 'dashboard/composables';
 import {
   MAXIMUM_FILE_UPLOAD_SIZE,
   MAXIMUM_FILE_UPLOAD_SIZE_TWILIO_SMS_CHANNEL,
@@ -38,13 +39,13 @@ export default {
 
         upload.create((error, blob) => {
           if (error) {
-            this.showAlert(error);
+            useAlert(error);
           } else {
             this.attachFile({ file, blob });
           }
         });
       } else {
-        this.showAlert(
+        useAlert(
           this.$t('CONVERSATION.FILE_SIZE_LIMIT', {
             MAXIMUM_SUPPORTED_FILE_UPLOAD_SIZE,
           })
@@ -61,7 +62,7 @@ export default {
       if (checkFileSizeLimit(file, MAXIMUM_SUPPORTED_FILE_UPLOAD_SIZE)) {
         this.attachFile({ file });
       } else {
-        this.showAlert(
+        useAlert(
           this.$t('CONVERSATION.FILE_SIZE_LIMIT', {
             MAXIMUM_SUPPORTED_FILE_UPLOAD_SIZE,
           })