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/i18n/locale/en/settings.json

@@ -694,7 +694,8 @@
         "CREATE_BUTTON": "Create policy",
         "API": {
           "SUCCESS_MESSAGE": "Assignment policy created successfully",
-          "ERROR_MESSAGE": "Failed to create assignment policy"
+          "ERROR_MESSAGE": "Failed to create assignment policy",
+          "INBOX_LINKED": "Inbox has been linked to the policy"
         }
       },
       "EDIT": {
@@ -708,6 +709,12 @@
           "CONFIRM_BUTTON_LABEL": "Continue",
           "CANCEL_BUTTON_LABEL": "Cancel"
         },
+        "INBOX_LINK_PROMPT": {
+          "TITLE": "Link inbox to policy",
+          "DESCRIPTION": "Would you like to link this inbox to the assignment policy?",
+          "LINK_BUTTON": "Link inbox",
+          "CANCEL_BUTTON": "Skip"
+        },
         "API": {
           "SUCCESS_MESSAGE": "Assignment policy updated successfully",
           "ERROR_MESSAGE": "Failed to update assignment policy"
@@ -746,7 +753,9 @@
           },
           "BALANCED": {
             "LABEL": "Balanced",
-            "DESCRIPTION": "Assign conversations based on available capacity."
+            "DESCRIPTION": "Assign conversations based on available capacity.",
+            "PREMIUM_MESSAGE": "Upgrade to access balanced assignment and agent capacity management.",
+            "PREMIUM_BADGE": "Premium"
           }
         },
         "ASSIGNMENT_PRIORITY": {
@@ -832,6 +841,20 @@
             "SUCCESS_MESSAGE": "Agent removed from policy successfully",
             "ERROR_MESSAGE": "Failed to remove agent from policy"
           }
+        },
+        "INBOX_LIMIT_API": {
+          "ADD": {
+            "SUCCESS_MESSAGE": "Inbox limit added successfully",
+            "ERROR_MESSAGE": "Failed to add inbox limit"
+          },
+          "UPDATE": {
+            "SUCCESS_MESSAGE": "Inbox limit updated successfully",
+            "ERROR_MESSAGE": "Failed to update inbox limit"
+          },
+          "DELETE": {
+            "SUCCESS_MESSAGE": "Inbox limit deleted successfully",
+            "ERROR_MESSAGE": "Failed to delete inbox limit"
+          }
         }
       },
       "FORM": {