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)