[Obs AI Assistant] Add aiAssistant:preferredAIAssistantType setting…

… to settings page (elastic#190969)

This adds the `aiAssistant:preferredAIAssistantType` setting (aka
"Observability AI Assistant scope") to the Obs AI Assistant settings
page. It was not necessary to re-define the setting definition as it was
already declared
[here](https://github.com/elastic/kibana/blob/5acd638327e5f22683910d2b97e05bbf4e87b3c9/src/plugins/ai_assistant_management/selection/server/plugin.ts#L48-L89)


![image](https://github.com/user-attachments/assets/96619f37-8835-4f49-8365-80b69f5d4699)

x-pack/plugins/observability_solution/observability_ai_assistant/common/ui_settings/settings_keys.ts

@@ -12,3 +12,4 @@ export const aiAssistantSimulatedFunctionCalling =
   'observability:aiAssistantSimulatedFunctionCalling';
 export const aiAssistantSearchConnectorIndexPattern =
   'observability:aiAssistantSearchConnectorIndexPattern';
+export const aiAssistantPreferredAIAssistantType = 'aiAssistant:preferredAIAssistantType';

x-pack/plugins/observability_solution/observability_ai_assistant/public/index.ts

@@ -100,6 +100,7 @@ export {
   aiAssistantLogsIndexPattern,
   aiAssistantSimulatedFunctionCalling,
   aiAssistantSearchConnectorIndexPattern,
+  aiAssistantPreferredAIAssistantType,
 } from '../common/ui_settings/settings_keys';
 
 export const plugin: PluginInitializer<

x-pack/plugins/observability_solution/observability_ai_assistant_management/public/routes/components/settings_tab/ui_settings.tsx

@@ -12,6 +12,7 @@ import {
   aiAssistantResponseLanguage,
   aiAssistantSimulatedFunctionCalling,
   aiAssistantSearchConnectorIndexPattern,
+  aiAssistantPreferredAIAssistantType,
 } from '@kbn/observability-ai-assistant-plugin/public';
 import { FieldRow, FieldRowProvider } from '@kbn/management-settings-components-field-row';
 import { EuiSpacer } from '@elastic/eui';
@@ -24,6 +25,7 @@ const settingsKeys = [
   aiAssistantResponseLanguage,
   aiAssistantSimulatedFunctionCalling,
   aiAssistantSearchConnectorIndexPattern,
+  aiAssistantPreferredAIAssistantType,
 ];
 
 export function UISettings() {