Fix error message for a put index template request without index_patterns (#27102)

Just correct the error message from "Validation Failed: 1: pattern is
missing;" to "Validation Failed: 1: index_patterns is missing;".

Closes #27100

core/src/main/java/org/elasticsearch/action/admin/indices/template/put/PutIndexTemplateRequest.java

@@ -104,7 +104,7 @@ public class PutIndexTemplateRequest extends MasterNodeRequest<PutIndexTemplateR
             validationException = addValidationError("name is missing", validationException);
         }
         if (indexPatterns == null || indexPatterns.size() == 0) {
-            validationException = addValidationError("pattern is missing", validationException);
+            validationException = addValidationError("index patterns are missing", validationException);
         }
         return validationException;
     }

core/src/test/java/org/elasticsearch/action/admin/indices/template/put/PutIndexTemplateRequestTests.java

@@ -19,6 +19,7 @@
 package org.elasticsearch.action.admin.indices.template.put;
 
 import org.elasticsearch.Version;
+import org.elasticsearch.action.ActionRequestValidationException;
 import org.elasticsearch.common.bytes.BytesArray;
 import org.elasticsearch.common.io.stream.BytesStreamOutput;
 import org.elasticsearch.common.io.stream.StreamInput;
@@ -32,6 +33,11 @@ import java.util.Arrays;
 import java.util.Base64;
 import java.util.Collections;
 
+import static org.hamcrest.Matchers.containsString;
+import static org.hamcrest.Matchers.hasSize;
+import static org.hamcrest.Matchers.nullValue;
+import static org.hamcrest.core.Is.is;
+
 public class PutIndexTemplateRequestTests extends ESTestCase {
 
     // bwc for #21009
@@ -107,4 +113,21 @@ public class PutIndexTemplateRequestTests extends ESTestCase {
             assertEquals("template", request.patterns().get(0));
         }
     }
+
+    public void testValidateErrorMessage() throws Exception {
+        PutIndexTemplateRequest request = new PutIndexTemplateRequest();
+        ActionRequestValidationException withoutNameAndPattern = request.validate();
+        assertThat(withoutNameAndPattern.getMessage(), containsString("name is missing"));
+        assertThat(withoutNameAndPattern.getMessage(), containsString("index patterns are missing"));
+
+        request.name("foo");
+        ActionRequestValidationException withoutIndexPatterns = request.validate();
+        assertThat(withoutIndexPatterns.validationErrors(), hasSize(1));
+        assertThat(withoutIndexPatterns.getMessage(), containsString("index patterns are missing"));
+
+        request.patterns(Collections.singletonList("test-*"));
+        ActionRequestValidationException noError = request.validate();
+        assertThat(noError, is(nullValue()));
+    }
+
 }

rest-api-spec/src/main/resources/rest-api-spec/test/indices.put_template/10_basic.yml

@@ -210,3 +210,12 @@
       catch: missing
       indices.get_template:
         name: "my_template"
+
+---
+"Put index template without index_patterns":
+
+  - do:
+      catch: /index patterns are missing/
+      indices.put_template:
+        name: test
+        body: {}