fix: guard agent sort against null names in assignment dropdown (#15125)

# Pull Request Template

## Description

This PR fixes a crash where opening a conversation threw `TypeError:
Cannot read properties of null (reading 'localeCompare')` and prevented
the agent assignment dropdown from rendering.

Since #14866, agent bots are included in the assignable agents list.
`AgentBot#name` is not presence-validated, so system bots (account-less,
global) can have a `null` name. Those nameless bots flowed into
name-based operations that assumed a string, causing crashes and
warnings across multiple surfaces:

* **Assignment dropdown sort:** `getAgentsByAvailability` called
`a.name.localeCompare(b.name)`, causing a `localeCompare` `TypeError`.
* **Dropdown search:** `MultiselectDropdownItems` called
`option.name.toLowerCase()`, causing a `toLowerCase` `TypeError`.
* **Agent Bots settings:** `Avatar` received `name=null` for a `String`
prop, triggering a Vue prop validation warning.

### What changed

* Keep nameless agent bots in the assignment dropdown and render a `-`
fallback label in `useAgentsList`. These are still valid,
assignable-by-ID records: the assignable agents API includes accessible
bots, and `Conversations::AssignmentService` assigns them by ID.
Preserving them avoids hiding valid assignment targets. Bots are still
included only when `includeAgentBots` is enabled.
* Make the sort in `getAgentsByAvailability` null-safe by coercing
missing names to an empty string (defense in depth).
* Make the search filter in `MultiselectDropdownItems` null-safe
(defense in depth).
* Pass a null-safe `name` prop to `Avatar` in the Agent Bots settings
list to eliminate the Vue prop validation warning.

Fixes
https://linear.app/chatwoot/issue/CW-7670/agent-assignment-dropdown-crashes-with-cannot-read-properties-of-null

## Type of change

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

## How Has This Been Tested?


1. Have a system agent bot (`name: null`) that is assignable to an
inbox.
2. Open any conversation in that inbox.
   * The agent assignment dropdown renders without console errors.
   * The nameless bot is listed with a `-` label and can be assigned.
3. Go to **Settings → Agent Bots**.
   * The page renders without the `Avatar` prop validation warning.


## 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/shared/components/ui/MultiselectDropdown.vue

@@ -68,6 +68,10 @@ const isAgentBot = computed(
   () => props.selectedItem?.assignee_type === 'AgentBot'
 );
 
+const selectedItemName = computed(() =>
+  !props.selectedItem?.name && isAgentBot.value ? '-' : props.selectedItem?.name
+);
+
 const selectedThumbnail = computed(
   () => props.selectedItem?.thumbnail || props.selectedItem?.avatar_url
 );
@@ -95,16 +99,16 @@ const selectedThumbnail = computed(
           <h4
             v-else
             class="items-center overflow-hidden text-sm leading-tight whitespace-nowrap text-ellipsis text-n-slate-12"
-            :title="selectedItem.name"
+            :title="selectedItemName"
           >
-            {{ selectedItem.name }}
+            {{ selectedItemName }}
           </h4>
         </div>
         <Avatar
           v-if="hasValue && hasThumbnail && (isAgentBot || !hasIcon)"
           :src="selectedThumbnail"
           :status="selectedItem.availability_status"
-          :name="selectedItem.name"
+          :name="selectedItemName"
           :icon-name="isAgentBot ? 'i-lucide-bot' : undefined"
           :size="24"
           hide-offline-status

app/javascript/shared/components/ui/MultiselectDropdownItems.vue

@@ -53,7 +53,9 @@ export default {
   computed: {
     filteredOptions() {
       return this.options.filter(option => {
-        return option.name.toLowerCase().includes(this.search.toLowerCase());
+        return (option.name || '')
+          .toLowerCase()
+          .includes(this.search.toLowerCase());
       });
     },
     noResult() {