feat: allow zero conversation limit capacity policy (#13964)

## Description

Two improvements to Agent Capacity Policy:

**1. Support exclusion via zero conversation limit**
Allow `conversation_limit` to be `0` on inbox capacity limits. Agents
with a zero limit are excluded from auto-assignment for that inbox while
remaining members for manual assignment.

**2. Fix exclusion rules duration input**
- Default changed from `10` to `null` so time-based exclusion isn't
applied unless explicitly set.
- Minimum lowered from 10 to 1 minute.
- `DurationInput` updated to handle `null` values correctly.

## Type of change

Please delete options that are not relevant.

- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)

## How Has This Been Tested?

- Added model and capacity service specs for zero-limit exclusion
behavior.
- Tested manually via UI flows 

## 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: Muhsin Keloth <muhsinkeramam@gmail.com>

spec/enterprise/models/inbox_capacity_limit_spec.rb

@@ -9,10 +9,22 @@ RSpec.describe InboxCapacityLimit, type: :model 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_numericality_of(:conversation_limit).is_greater_than_or_equal_to(0).only_integer }
     it { is_expected.to validate_uniqueness_of(:inbox_id).scoped_to(:agent_capacity_policy_id) }
   end
 
+  describe 'zero conversation limit (exclusion policy)' do
+    it 'allows conversation_limit of 0' do
+      limit = build(:inbox_capacity_limit, agent_capacity_policy: policy, inbox: inbox, conversation_limit: 0)
+      expect(limit).to be_valid
+    end
+
+    it 'rejects negative conversation_limit' do
+      limit = build(:inbox_capacity_limit, agent_capacity_policy: policy, inbox: inbox, conversation_limit: -1)
+      expect(limit).not_to be_valid
+    end
+  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)

spec/enterprise/services/enterprise/auto_assignment/capacity_service_spec.rb

@@ -86,6 +86,55 @@ RSpec.describe Enterprise::AutoAssignment::CapacityService, type: :service do
     end
   end
 
+  describe 'exclusion policy (zero conversation limit)' do
+    let(:excluded_agent) { create(:user, account: account, role: :agent, availability: :online) }
+    let(:exclusion_policy) { create(:agent_capacity_policy, account: account, name: 'Exclusion Policy') }
+
+    before do
+      create(:inbox_capacity_limit,
+             agent_capacity_policy: exclusion_policy,
+             inbox: inbox,
+             conversation_limit: 0)
+
+      excluded_agent.account_users.find_by(account: account)
+                    .update!(agent_capacity_policy: exclusion_policy)
+
+      create(:inbox_member, inbox: inbox, user: excluded_agent)
+
+      allow(OnlineStatusTracker).to receive(:get_available_users).and_return({
+                                                                               excluded_agent.id.to_s => 'online',
+                                                                               agent_with_capacity.id.to_s => 'online',
+                                                                               agent_without_capacity.id.to_s => 'online',
+                                                                               agent_at_capacity.id.to_s => 'online'
+                                                                             })
+    end
+
+    it 'always denies capacity for agents with zero limit' do
+      capacity_service = described_class.new
+      expect(capacity_service.agent_has_capacity?(excluded_agent, inbox)).to be false
+    end
+
+    it 'denies capacity even when agent has no existing conversations' do
+      capacity_service = described_class.new
+      # Agent has 0 open conversations but limit is 0, so 0 < 0 is false
+      expect(excluded_agent.assigned_conversations.where(inbox: inbox, status: :open).count).to eq(0)
+      expect(capacity_service.agent_has_capacity?(excluded_agent, inbox)).to be false
+    end
+
+    it 'excludes zero-limit agents from available agents list' do
+      capacity_service = described_class.new
+      online_agents = inbox.available_agents
+      filtered_agents = online_agents.select do |inbox_member|
+        capacity_service.agent_has_capacity?(inbox_member.user, inbox)
+      end
+      available_users = filtered_agents.map(&:user)
+
+      expect(available_users).not_to include(excluded_agent)
+      expect(available_users).to include(agent_with_capacity)
+      expect(available_users).to include(agent_without_capacity)
+    end
+  end
+
   describe 'assignment with capacity' do
     let(:service) { AutoAssignment::AssignmentService.new(inbox: inbox) }