Throw 4xx instead of 5xx for ESQL malformed async query params (elastic#134879)

Author: pmpailis

docs/changelog/134879.yaml

@@ -0,0 +1,6 @@
+pr: 134879
+summary: Throw 4xx instead of 5xx for ESQL malformed query params
+area: ES|QL
+type: bug
+issues:
+ - 134618

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/action/RequestXContent.java

@@ -222,6 +222,18 @@ private static QueryParams parseParams(XContentParser p) throws IOException {
                     }
                 }
             }
+        } else {
+            errors.add(
+                new XContentParseException(
+                    "Unexpected token ["
+                        + token
+                        + "] at "
+                        + p.getTokenLocation()
+                        + ", expected "
+                        + XContentParser.Token.START_ARRAY
+                        + ". Please check documentation for the correct format of the 'params' field."
+                )
+            );
         }
         // don't allow mixed named and unnamed parameters
         if (namedParams.isEmpty() == false && unNamedParams.isEmpty() == false) {

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/action/EsqlQueryRequestTests.java

@@ -408,6 +408,26 @@ public void testInvalidMultivaluedUnnamedParams() throws IOException {
         );
     }
 
+    public void testInvalidParamsString() {
+        String query = randomAlphaOfLengthBetween(1, 100);
+        String json1 = String.format(Locale.ROOT, """
+            {
+                "query": "%s",
+                "params": {*}
+            }""", query);
+        Exception e1 = expectThrows(XContentParseException.class, () -> parseEsqlQueryRequestSync(json1));
+        String message1 = e1.getCause().getMessage();
+        assertThat("Unexpected failure when parsing " + json1 + ". " + message1, containsString("Unexpected token [START_OBJECT]"));
+        String json2 = String.format(Locale.ROOT, """
+            {
+                "query": "%s",
+                "params": "foo"
+            }""", query);
+        Exception e2 = expectThrows(XContentParseException.class, () -> parseEsqlQueryRequestSync(json2));
+        String message2 = e2.getCause().getMessage();
+        assertThat("Unexpected failure when parsing " + json2 + ". " + message2, containsString("Unexpected token [VALUE_STRING]"));
+    }
+
     public void testInvalidParamsForIdentifiersPatterns() throws IOException {
         String query = randomAlphaOfLengthBetween(1, 100);
         boolean columnar = randomBoolean();