[Obs AI Assistant] Fix AI Assistant Settings when plugin disabled (el…

…astic#188160)

## Summary

Fixes elastic#187977

AI Assistant settings page doesn't load when the plugin is disabled.

### Changes

- fixes not checking for object before accessing property
- simplifies message so it makes sense for all possible states (read
only, disabled in space, plugin disabled)

### Testing steps
- create a Viewer user
- go to Stack Management -> Advanced Settings -> AI Assistant
- page should load with message that the feature is disabled

<img width="804" alt="Screenshot 2024-07-11 at 4 57 59 PM"
src="https://github.com/user-attachments/assets/98cf85a2-913f-43fd-a5f0-df4494faaf1f">

(cherry picked from commit d4ba879)

src/plugins/ai_assistant_management/selection/public/routes/components/ai_assistant_selection_page.tsx

@@ -24,8 +24,7 @@ import { useAppContext } from '../../app_context';
 
 export function AiAssistantSelectionPage() {
   const { capabilities, setBreadcrumbs, navigateToApp } = useAppContext();
-
-  const observabilityAIAssistantEnabled = capabilities.observabilityAIAssistant.show;
+  const observabilityAIAssistantEnabled = capabilities.observabilityAIAssistant?.show;
   const securityAIAssistantEnabled = capabilities.securitySolutionAssistant?.['ai-assistant'];
 
   useEffect(() => {
@@ -78,8 +77,7 @@ export function AiAssistantSelectionPage() {
                       title={i18n.translate(
                         'aiAssistantManagementSelection.aiAssistantSelectionPage.observabilityAi.thisFeatureIsDisabledCallOutLabel',
                         {
-                          defaultMessage:
-                            'This feature is disabled. It can be enabled from Spaces > Features.',
+                          defaultMessage: 'This feature is disabled.',
                         }
                       )}
                       size="s"