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/routes/dashboard/settings/assignmentPolicy/pages/AgentCapacityEditPage.vue

@@ -81,7 +81,7 @@ const formData = computed(() => ({
       ...(selectedPolicy.value?.exclusionRules?.excludedLabels || []),
     ],
     excludeOlderThanHours:
-      selectedPolicy.value?.exclusionRules?.excludeOlderThanHours || 10,
+      selectedPolicy.value?.exclusionRules?.excludeOlderThanHours ?? null,
   },
   inboxCapacityLimits:
     selectedPolicy.value?.inboxCapacityLimits?.map(limit => ({

app/javascript/dashboard/routes/dashboard/settings/assignmentPolicy/pages/components/AgentCapacityPolicyForm.vue

@@ -17,7 +17,7 @@ const props = defineProps({
       enabled: false,
       exclusionRules: {
         excludedLabels: [],
-        excludeOlderThanHours: 10,
+        excludeOlderThanHours: null,
       },
       inboxCapacityLimits: [],
     }),
@@ -84,7 +84,7 @@ const state = reactive({
   description: '',
   exclusionRules: {
     excludedLabels: [],
-    excludeOlderThanHours: 10,
+    excludeOlderThanHours: null,
   },
   inboxCapacityLimits: [],
 });
@@ -120,7 +120,7 @@ const resetForm = () => {
     description: '',
     exclusionRules: {
       excludedLabels: [],
-      excludeOlderThanHours: 10,
+      excludeOlderThanHours: null,
     },
     inboxCapacityLimits: [],
   });