fix: Translate "None" option in agent assignment dropdown (#11060)

# Pull Request Template

## Description

This PR includes a translation update for the "None" option in the agent
assignment multi-select dropdown.

Fixes
https://linear.app/chatwoot/issue/CW-4140/none-option-in-assign-agent-multi-select-is-not-translated

## Type of change

- [x] Bug fix (non-breaking change which fixes an issue)

## How Has This Been Tested?

**Test cases**
1. Check in conversation sidebar
2. Check in command bar
3. Check in participation dropdown


## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

app/javascript/dashboard/i18n/locale/en/agentMgmt.json

@@ -105,6 +105,9 @@
         "AGENT": "Select agent",
         "TEAM": "Select team"
       },
+      "LIST": {
+        "NONE": "None"
+      },
       "SEARCH": {
         "NO_RESULTS": {
           "AGENT": "No agents found",