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>

spec/enterprise/models/agent_capacity_policy_spec.rb

@@ -0,0 +1,29 @@
+require 'rails_helper'
+
+RSpec.describe AgentCapacityPolicy, type: :model do
+  let(:account) { create(:account) }
+
+  describe 'validations' do
+    it { is_expected.to validate_presence_of(:name) }
+    it { is_expected.to validate_length_of(:name).is_at_most(255) }
+  end
+
+  describe 'destruction' do
+    let(:policy) { create(:agent_capacity_policy, account: account) }
+    let(:user) { create(:user, account: account) }
+    let(:inbox) { create(:inbox, account: account) }
+
+    it 'destroys associated inbox capacity limits' do
+      create(:inbox_capacity_limit, agent_capacity_policy: policy, inbox: inbox)
+      expect { policy.destroy }.to change(InboxCapacityLimit, :count).by(-1)
+    end
+
+    it 'nullifies associated account users' do
+      account_user = user.account_users.first
+      account_user.update!(agent_capacity_policy: policy)
+
+      policy.destroy
+      expect(account_user.reload.agent_capacity_policy).to be_nil
+    end
+  end
+end

spec/enterprise/models/inbox_capacity_limit_spec.rb

@@ -0,0 +1,33 @@
+require 'rails_helper'
+
+RSpec.describe InboxCapacityLimit, type: :model do
+  let(:account) { create(:account) }
+  let(:policy) { create(:agent_capacity_policy, account: account) }
+  let(:inbox) { create(:inbox, account: account) }
+
+  describe 'validations' do
+    subject { create(:inbox_capacity_limit, agent_capacity_policy: policy, inbox: inbox) }
+
+    it { is_expected.to validate_presence_of(:conversation_limit) }
+    it { is_expected.to validate_numericality_of(:conversation_limit).is_greater_than(0).only_integer }
+    it { is_expected.to validate_uniqueness_of(:inbox_id).scoped_to(:agent_capacity_policy_id) }
+  end
+
+  describe 'uniqueness constraint' do
+    it 'prevents duplicate inbox limits for the same policy' do
+      create(:inbox_capacity_limit, agent_capacity_policy: policy, inbox: inbox)
+      duplicate = build(:inbox_capacity_limit, agent_capacity_policy: policy, inbox: inbox)
+
+      expect(duplicate).not_to be_valid
+      expect(duplicate.errors[:inbox_id]).to include('has already been taken')
+    end
+
+    it 'allows the same inbox in different policies' do
+      other_policy = create(:agent_capacity_policy, account: account)
+      create(:inbox_capacity_limit, agent_capacity_policy: policy, inbox: inbox)
+
+      different_policy_limit = build(:inbox_capacity_limit, agent_capacity_policy: other_policy, inbox: inbox)
+      expect(different_policy_limit).to be_valid
+    end
+  end
+end