fix: V2 Assignment service enhancements (#13036)

## Linear Ticket:
https://linear.app/chatwoot/issue/CW-6081/review-feedback

## Description

Assignment V2 Service Enhancements

- Enable Assignment V2 on plan upgrade
- Fix UI issue with fair distribution policy display
- Add advanced assignment feature flag and enhance Assignment V2
capabilities

## Type of change

- [ ] Bug fix (non-breaking change which fixes an issue)

## How Has This Been Tested?

This has been tested using the UI.

## Checklist:

- [ ] My code follows the style guidelines of this project
- [ ] 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
- [ ] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Changes auto-assignment execution paths, rate limiting defaults, and
feature-flag gating (including premium plan behavior), which could
affect which conversations get assigned and when. UI rewires inbox
settings and policy flows, so regressions are possible around
navigation/linking and feature visibility.
> 
> **Overview**
> **Adds a new premium `advanced_assignment` feature flag** and uses it
to gate capacity/balanced assignment features in the UI (sidebar entry,
settings routes, assignment-policy landing cards) and backend
(Enterprise balanced selector + capacity filtering).
`advanced_assignment` is marked premium, included in Business plan
entitlements, and auto-synced in Enterprise accounts when
`assignment_v2` is toggled.
> 
> **Improves Assignment V2 policy UX** by adding an inbox-level
“Conversation Assignment” section (behind `assignment_v2`) that can
link/unlink an assignment policy, navigate to create/edit policy flows
with `inboxId` query context, and show an inbox-link prompt after
creating a policy. The policy form now defaults to enabled, disables the
`balanced` option with a premium badge/message when unavailable, and
inbox lists support click-to-navigate.
> 
> **Tightens/adjusts auto-assignment behavior**: bulk assignment now
requires `inbox.enable_auto_assignment?`, conversation ordering uses the
attached `assignment_policy` priority, and rate limiting uses
`assignment_policy` config with an infinite default limit while still
tracking assignments. Tests and i18n strings are updated accordingly.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
23bc03bf75ee4376071e4d7fc7cd564c601d33d7. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Pranav <pranav@chatwoot.com>
Co-authored-by: iamsivin <iamsivin@gmail.com>
Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>
Co-authored-by: Shivam Mishra <scm.mymail@gmail.com>

app/javascript/dashboard/components-next/sidebar/Sidebar.vue

@@ -8,6 +8,7 @@ import { useStore } from 'vuex';
 import { useI18n } from 'vue-i18n';
 import { useSidebarKeyboardShortcuts } from './useSidebarKeyboardShortcuts';
 import { vOnClickOutside } from '@vueuse/components';
+import { FEATURE_FLAGS } from 'dashboard/featureFlags';
 import { useWindowSize, useEventListener } from '@vueuse/core';
 import { emitter } from 'shared/helpers/mitt';
 import { BUS_EVENTS } from 'shared/constants/busEvents';
@@ -50,6 +51,18 @@ const isRTL = useMapGetter('accounts/isRTL');
 const { width: windowWidth } = useWindowSize();
 const isMobile = computed(() => windowWidth.value < 768);
 
+const accountId = useMapGetter('getCurrentAccountId');
+const isFeatureEnabledonAccount = useMapGetter(
+  'accounts/isFeatureEnabledonAccount'
+);
+
+const hasAdvancedAssignment = computed(() => {
+  return isFeatureEnabledonAccount.value(
+    accountId.value,
+    FEATURE_FLAGS.ADVANCED_ASSIGNMENT
+  );
+});
+
 const toggleShortcutModalFn = show => {
   if (show) {
     emit('openKeyShortcutModal');
@@ -584,12 +597,16 @@ const menuItems = computed(() => {
           icon: 'i-lucide-users',
           to: accountScopedRoute('settings_teams_list'),
         },
-        {
-          name: 'Settings Agent Assignment',
-          label: t('SIDEBAR.AGENT_ASSIGNMENT'),
-          icon: 'i-lucide-user-cog',
-          to: accountScopedRoute('assignment_policy_index'),
-        },
+        ...(hasAdvancedAssignment.value
+          ? [
+              {
+                name: 'Settings Agent Assignment',
+                label: t('SIDEBAR.AGENT_ASSIGNMENT'),
+                icon: 'i-lucide-user-cog',
+                to: accountScopedRoute('assignment_policy_index'),
+              },
+            ]
+          : []),
         {
           name: 'Settings Inboxes',
           label: t('SIDEBAR.INBOXES'),