feat: Rewrite agentMixin to a helper (#9940)

# Pull Request Template

## Description

This PR will replace the usage of `agentMixin`with the utility helpers
functions.

**Files updated**

1. dashboard/components/widgets/conversation/contextMenu/Index.vue
2. dashboard/components/widgets/conversation/ConversationHeader.vue
**(Not used)**
3. dashboard/routes/dashboard/commands/commandbar.vue
4. dashboard/routes/dashboard/conversation/ConversationAction.vue
5. dashboard/routes/dashboard/conversation/ConversationParticipant.vue

Fixes
https://linear.app/chatwoot/issue/CW-3442/rewrite-agentmixin-mixin-to-a-composable

## Type of change

- [x] New feature (non-breaking change which adds functionality)

## How Has This Been Tested?

**Test cases**

1. See agent list sorting based on availability, if agents are on the
same status, then sorted by name.
2. Test actions like assigning/unassigning agent from conversation
sidebar, CMD bar, Context menu.
3. Test actions like adding/removing participants from conversation
sidebar.
4. See agent list is generated properly, none value.


## Checklist:

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

app/javascript/dashboard/components/widgets/conversation/ConversationHeader.vue

@@ -2,7 +2,6 @@
 import { ref } from 'vue';
 import { mapGetters } from 'vuex';
 import { useKeyboardEvents } from 'dashboard/composables/useKeyboardEvents';
-import agentMixin from '../../../mixins/agentMixin.js';
 import BackButton from '../BackButton.vue';
 import inboxMixin from 'shared/mixins/inboxMixin';
 import InboxName from '../InboxName.vue';
@@ -24,7 +23,7 @@ export default {
     SLACardLabel,
     Linear,
   },
-  mixins: [inboxMixin, agentMixin],
+  mixins: [inboxMixin],
   props: {
     chat: {
       type: Object,