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>

app/javascript/dashboard/components-next/AssignmentPolicy/components/ExclusionRules.vue

@@ -20,11 +20,11 @@ const excludedLabels = defineModel('excludedLabels', {
 
 const excludeOlderThanMinutes = defineModel('excludeOlderThanMinutes', {
   type: Number,
-  default: 10,
+  default: null,
 });
 
-// Duration limits: 10 minutes to 999 days (in minutes)
-const MIN_DURATION_MINUTES = 10;
+// Duration limits: 1 minute to 999 days (in minutes)
+const MIN_DURATION_MINUTES = 1;
 const MAX_DURATION_MINUTES = 1438560; // 999 days * 24 hours * 60 minutes
 
 const { t } = useI18n();