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/composables/spec/fixtures/agentFixtures.js

@@ -0,0 +1,63 @@
+import { allAgentsData } from 'dashboard/helper/specs/fixtures/agentFixtures';
+
+export { allAgentsData };
+export const formattedAgentsData = [
+  {
+    account_id: 0,
+    confirmed: true,
+    email: 'None',
+    id: 0,
+    name: 'None',
+    role: 'agent',
+  },
+  {
+    account_id: 1,
+    availability_status: 'online',
+    available_name: 'Abraham',
+    confirmed: true,
+    email: 'abraham@chatwoot.com',
+    id: 5,
+    name: 'Abraham Keta',
+    role: 'agent',
+  },
+  {
+    account_id: 1,
+    availability_status: 'online',
+    available_name: 'John K',
+    confirmed: true,
+    email: 'john@chatwoot.com',
+    id: 1,
+    name: 'John Kennady',
+    role: 'administrator',
+  },
+  {
+    account_id: 1,
+    availability_status: 'busy',
+    available_name: 'Honey',
+    confirmed: true,
+    email: 'bee@chatwoot.com',
+    id: 4,
+    name: 'Honey Bee',
+    role: 'agent',
+  },
+  {
+    account_id: 1,
+    availability_status: 'busy',
+    available_name: 'Samuel K',
+    confirmed: true,
+    email: 'samuel@chatwoot.com',
+    id: 2,
+    name: 'Samuel Keta',
+    role: 'agent',
+  },
+  {
+    account_id: 1,
+    availability_status: 'offline',
+    available_name: 'James K',
+    confirmed: true,
+    email: 'james@chatwoot.com',
+    id: 3,
+    name: 'James Koti',
+    role: 'agent',
+  },
+];