feat: Add agent capacity controllers (#12200)

## Linear reference:
https://linear.app/chatwoot/issue/CW-4649/re-imagine-assignments

## Description
This PR introduces the foundation for Assignment V2 system by
implementing agent_capacity and their association with inboxes and
users.

## Type of change

- [ ] New feature (non-breaking change which adds functionality)

## How Has This Been Tested?

Test Coverage:
-  Controller specs for assignment policies CRUD operations
-  Enterprise-specific specs for balanced assignment order
-  Model specs for community/enterprise separation

## 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

---------

Co-authored-by: Pranav <pranav@chatwoot.com>

enterprise/app/views/api/v1/models/_agent_capacity_policy.json.jbuilder

@@ -0,0 +1,13 @@
+json.id agent_capacity_policy.id
+json.name agent_capacity_policy.name
+json.description agent_capacity_policy.description
+json.exclusion_rules agent_capacity_policy.exclusion_rules
+json.created_at agent_capacity_policy.created_at.to_i
+json.updated_at agent_capacity_policy.updated_at.to_i
+json.account_id agent_capacity_policy.account_id
+
+json.inbox_capacity_limits agent_capacity_policy.inbox_capacity_limits do |limit|
+  json.id limit.id
+  json.inbox_id limit.inbox_id
+  json.conversation_limit limit.conversation_limit
+end