Explicit check for retriever object

Author: mridula-s109

x-pack/plugin/rank-rrf/src/main/java/org/elasticsearch/xpack/rank/rrf/RRFRetrieverComponent.java

@@ -82,6 +82,9 @@ public static RRFRetrieverComponent fromXContent(XContentParser parser, Retrieve
                         throw new ParsingException(parser.getTokenLocation(), "only one retriever can be specified");
                     }
                     parser.nextToken();
+                    if (parser.currentToken() != XContentParser.Token.START_OBJECT) {
+                        throw new ParsingException(parser.getTokenLocation(), "retriever must be an object");
+                    }
                     parser.nextToken();
                     String retrieverType = parser.currentName();
                     retriever = parser.namedObject(RetrieverBuilder.class, retrieverType, context);

x-pack/plugin/rank-rrf/src/test/java/org/elasticsearch/xpack/rank/rrf/RRFRetrieverBuilderParsingTests.java

@@ -336,6 +336,27 @@ public void testRRFRetrieverComponentErrorCases() throws IOException {
             """;
 
         expectParsingException(negativeWeightContent, "[weight] must be non-negative");
+
+        // Test case 5: Retriever as non-object
+        String retrieverAsStringContent = """
+            {
+              "retriever": {
+                "rrf": {
+                  "retrievers": [
+                    {
+                      "retriever": "not_an_object"
+                    }
+                  ],
+                  "rank_window_size": 100,
+                  "rank_constant": 10,
+                  "min_score": 20.0,
+                  "_name": "foo_rrf"
+                }
+              }
+            }
+            """;
+
+        expectParsingException(retrieverAsStringContent, "retriever must be an object");
     }
 
     private void expectParsingException(String restContent, String expectedMessageFragment) throws IOException {