Fix bwc

Author: kanoshiou

x-pack/plugin/esql/qa/testFixtures/src/main/resources/eval.csv-spec

@@ -637,6 +637,7 @@ foo:keyword
 ;
 
 evalAfterAvgGroupingUsingSameName
+required_capability: retain_aggregate_when_grouping
 from employees
 | stats avg = avg(salary) by gender 
 | keep avg 
@@ -650,6 +651,7 @@ avg:integer
 ;
 
 evalAfterGroupingUsingSameName
+required_capability: retain_aggregate_when_grouping
 row foo = [10,11,9], bar = [1,2,3]
 | mv_expand foo
 | mv_expand bar 
@@ -665,6 +667,7 @@ this:integer
 ;
 
 evalAfterGroupingUsingSameName2
+required_capability: retain_aggregate_when_grouping
 from employees 
 | stats count = count(emp_no) by gender, is_rehired 
 | keep count 

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

@@ -413,6 +413,12 @@ public enum Cap {
          */
         RENAME_SEQUENTIAL_PROCESSING,
 
+        /**
+         * Support for retain aggregate when grouping.
+         * See <a href="https://github.com/elastic/elasticsearch/issues/126026"> ES|QL: columns not projected away despite KEEP #126026 </a>
+         */
+        RETAIN_AGGREGATE_WHEN_GROUPING,
+
         /**
          * Fix for union-types when some indexes are missing the required field. Done in #111932.
          */