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/useAgentsList.js

@@ -0,0 +1,57 @@
+import { computed } from 'vue';
+import { useMapGetter } from 'dashboard/composables/store';
+import {
+  getAgentsByUpdatedPresence,
+  getSortedAgentsByAvailability,
+  getCombinedAgents,
+} from 'dashboard/helper/agentHelper';
+
+/**
+ * A composable function that provides a list of agents for assignment.
+ *
+ * @param {boolean} [includeNoneAgent=true] - Whether to include a 'None' agent option.
+ * @returns {Object} An object containing the agents list and assignable agents.
+ */
+export function useAgentsList(includeNoneAgent = true) {
+  const currentUser = useMapGetter('getCurrentUser');
+  const currentChat = useMapGetter('getSelectedChat');
+  const currentAccountId = useMapGetter('getCurrentAccountId');
+  const assignable = useMapGetter('inboxAssignableAgents/getAssignableAgents');
+
+  const inboxId = computed(() => currentChat.value?.inbox_id);
+  const isAgentSelected = computed(() => currentChat.value?.meta?.assignee);
+
+  /**
+   * @type {import('vue').ComputedRef<Array>}
+   */
+  const assignableAgents = computed(() => {
+    return inboxId.value ? assignable.value(inboxId.value) : [];
+  });
+
+  /**
+   * @type {import('vue').ComputedRef<Array>}
+   */
+  const agentsList = computed(() => {
+    const agents = assignableAgents.value || [];
+    const agentsByUpdatedPresence = getAgentsByUpdatedPresence(
+      agents,
+      currentUser.value,
+      currentAccountId.value
+    );
+
+    const filteredAgentsByAvailability = getSortedAgentsByAvailability(
+      agentsByUpdatedPresence
+    );
+
+    return getCombinedAgents(
+      filteredAgentsByAvailability,
+      includeNoneAgent,
+      isAgentSelected.value
+    );
+  });
+
+  return {
+    agentsList,
+    assignableAgents,
+  };
+}