Add Autocomplete for Diversify Retriever (#244411)

## Summary

Adds console autocomplete for the diversify retriever in DSL.This was
added into Elasticsearch here:
elastic/elasticsearch#135880

## Screenshot

<img width="637" height="333" alt="image"
src="https://github.com/user-attachments/assets/3028929c-1353-49d8-840a-a7a0c38a6302"
/>

Author: markjhoy

src/platform/plugins/shared/console/server/lib/spec_definitions/js/retriever.ts

@@ -12,6 +12,25 @@ import { ChunkingSettings } from './shared';
 
 export const retriever = (specService: SpecDefinitionsService) => {
   specService.addGlobalAutocompleteRules('retriever', {
+    diversify: {
+      __template: {
+        type: '',
+        field: '',
+        retriever: {},
+      },
+      // only `mmr` is available at the moment. More to come in the future.
+      type: { __one_of: ['mmr'] },
+      field: '{field}',
+      retriever: {
+        __scope_link: '.',
+      },
+      // size is only applicable for 'mmr' diversify type
+      size: 10,
+      rank_window_size: 100,
+      query_vector: [],
+      // lambda is only applicable for 'mmr' diversify type
+      lambda: 0.5,
+    },
     knn: {
       __template: {
         field: '',