fix: resolve V2 capacity bypass in team assignment (#13904)

## Description

When Assignment V2 is enabled, the V2 capacity policies
(AgentCapacityPolicy / InboxCapacityLimit) are not respected during
team-based assignment paths. The system falls back to the legacy V1
max_assignment_limit, and since V1 is deprecated and typically
unconfigured in V2 setups, agents receive unlimited assignments
regardless of their V2 capacity.

Root cause: Inbox class directly defined
member_ids_with_assignment_capacity, which shadowed the
Enterprise::InboxAgentAvailability module override in Ruby's method
resolution order (MRO). This made the V2 capacity check unreachable
(dead code) for any code path using member_ids_with_assignment_capacity.

## Type of change

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

## How Has This Been Tested?

⏺ Before the fix
1. Enable assignment_v2 + advanced_assignment on account
2. Create AgentCapacityPolicy with InboxCapacityLimit = 1 for an inbox
3. Assign the policy to an agent (e.g., John)
4. Create 1 open conversation assigned to John (now at capacity)
5. Create a new unassigned conversation in the same inbox
6. Assign a team (containing John) to that conversation
7. Result: John gets assigned despite being at capacity
⏺ After the fix
Same steps 1–6.
7. Result: John is NOT assigned — conversation stays unassigned (no
agents with capacity available)


## 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

enterprise/app/models/enterprise/inbox.rb

@@ -1,6 +1,7 @@
 module Enterprise::Inbox
   def member_ids_with_assignment_capacity
     return super unless enable_auto_assignment?
+    return filter_by_capacity(available_agents).map(&:user_id) if auto_assignment_v2_enabled?
 
     max_assignment_limit = auto_assignment_config['max_assignment_limit']
     overloaded_agent_ids = max_assignment_limit.present? ? get_agent_ids_over_assignment_limit(max_assignment_limit) : []

enterprise/app/models/enterprise/inbox_agent_availability.rb

@@ -21,7 +21,7 @@ module Enterprise::InboxAgentAvailability
   end
 
   def capacity_filtering_enabled?
-    account.feature_enabled?('assignment_v2') &&
+    account.feature_enabled?('advanced_assignment') &&
       account.account_users.joins(:agent_capacity_policy).exists?
   end