Add capability for rejecting invalid nesting aggs

Author: MattAlp

modules/aggregations/src/yamlRestTest/resources/rest-api-spec/test/aggregations/nested.yml

@@ -196,6 +196,13 @@ setup:
   - match: { aggregations.courses.highpass_filter.unnest.department.buckets.1.doc_count: 1 }
 ---
 "Illegal reverse nested aggregation to a child nested object":
+  - requires:
+        capabilities:
+          - method: POST
+            path: /_search
+            capabilities: [ reject_invalid_reverse_nesting ]
+        test_runner_features: [ capabilities ]
+        reason: "search does not yet reject invalid reverse nesting paths"
   - do:
       catch: /Reverse nested path \[courses.sessions\] is not a parent of the current nested scope \[courses\]/
       search:

server/src/main/java/org/elasticsearch/rest/action/search/SearchCapabilities.java

@@ -60,6 +60,7 @@ private SearchCapabilities() {}
     private static final String BUCKET_SCRIPT_PARENT_MULTI_BUCKET_ERROR = "bucket_script_parent_multi_bucket_error";
     private static final String EXCLUDE_SOURCE_VECTORS_SETTING = "exclude_source_vectors_setting";
     private static final String CLUSTER_STATS_EXTENDED_USAGE = "extended-search-usage-stats";
+    private static final String REJECT_INVALID_REVERSE_NESTING = "reject_invalid_reverse_nesting";
 
     public static final Set<String> CAPABILITIES;
     static {
@@ -90,6 +91,7 @@ private SearchCapabilities() {}
         capabilities.add(BUCKET_SCRIPT_PARENT_MULTI_BUCKET_ERROR);
         capabilities.add(EXCLUDE_SOURCE_VECTORS_SETTING);
         capabilities.add(CLUSTER_STATS_EXTENDED_USAGE);
+        capabilities.add(REJECT_INVALID_REVERSE_NESTING);
         CAPABILITIES = Set.copyOf(capabilities);
     }
 }