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/components/widgets/conversation/ReplyBox.vue

@@ -153,7 +153,7 @@
 
 <script>
 import { mapGetters } from 'vuex';
-import alertMixin from 'shared/mixins/alertMixin';
+import { useAlert } from 'dashboard/composables';
 import keyboardEventListenerMixins from 'shared/mixins/keyboardEventListenerMixins';
 
 import CannedResponse from './CannedResponse.vue';
@@ -219,7 +219,6 @@ export default {
   mixins: [
     inboxMixin,
     uiSettingsMixin,
-    alertMixin,
     messageFormatterMixin,
     rtlMixin,
     fileUploadMixin,
@@ -310,7 +309,7 @@ export default {
             agentId,
           })
           .then(() => {
-            this.showAlert(this.$t('CONVERSATION.CHANGE_AGENT'));
+            useAlert(this.$t('CONVERSATION.CHANGE_AGENT'));
           });
       },
     },
@@ -880,7 +879,7 @@ export default {
       } catch (error) {
         const errorMessage =
           error?.response?.data?.error || this.$t('CONVERSATION.MESSAGE_ERROR');
-        this.showAlert(errorMessage);
+        useAlert(errorMessage);
       }
     },
     async onSendWhatsAppReply(messagePayload) {