ES|QL: Polish the code.

Polish the TBUCKET related code.

Author: leontyevdv

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

@@ -1340,7 +1340,7 @@ public enum Cap {
         /**
          * Support for tbucket function
          */
-        TBUCKET();
+        TBUCKET;
 
         private final boolean enabled;
 

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/analysis/VerifierTests.java

@@ -2521,6 +2521,11 @@ public void testInvalidTBucketCalls() {
             error("from test | stats max(event_duration) by tbucket(\"1\")", sampleDataAnalyzer),
             equalTo("1:42: argument of [tbucket(\"1\")] must be [date_period or time_duration], found value [\"1\"] type [keyword]")
         );
+
+        /*
+        To test unsupported @timestamp data type. In this case, we use a boolean as a type for the @timestamp field which is not supported
+        by TBUCKET.
+         */
         assertThat(
             error("from test | stats max(event_duration) by tbucket(\"1 hour\")", oddSampleDataAnalyzer),
             equalTo(