Add capability for bwc tests

Author: bpintea

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

@@ -426,6 +426,7 @@ emp_no:integer |first_name:keyword
 
 # test for https://github.com/elastic/elasticsearch/issues/128813
 rlikeWithEmptyLanguagePattern
+required_capability: rlike_with_empty_language_pattern
 ROW x = "abc" | EVAL bool = x RLIKE "#"
 ;
 

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

@@ -1164,7 +1164,12 @@ public enum Cap {
         /**
          * Enable support for index aliases in lookup joins
          */
-        ENABLE_LOOKUP_JOIN_ON_ALIASES(JOIN_LOOKUP_V12.isEnabled());
+        ENABLE_LOOKUP_JOIN_ON_ALIASES(JOIN_LOOKUP_V12.isEnabled()),
+
+        /**
+         * Allows RLIKE to correctly handle the "empty language" flag, `#`.
+         */
+        RLIKE_WITH_EMPTY_LANGUAGE_PATTERN;
 
         private final boolean enabled;