Fix flaky ExistsQueryBuilderTests.testToQuery (opensearch-project#18995)

* Fix flaky ExistsQueryBuilderTests.testToQuery

Test was generating field patterns that matched raw.derived_keyword,
which doesnt support exists queries. Fixed by replacing problematic
patterns with TEXT_FIELD_NAME.

Fixes opensearch-project#18724

Signed-off-by: Atri Sharma <atri.jiit@gmail.com>

* Revert CHANGELOG changes

Signed-off-by: Atri Sharma <atri.jiit@gmail.com>

---------

Signed-off-by: Atri Sharma <atri.jiit@gmail.com>

Author: atris

server/src/test/java/org/opensearch/index/query/ExistsQueryBuilderTests.java

@@ -87,12 +87,20 @@ protected ExistsQueryBuilder doCreateTestQueryBuilder() {
         } else {
             fieldPattern = randomAlphaOfLengthBetween(1, 10);
         }
+
+        // Avoid patterns that would match the derived field "raw.derived_keyword"
+        // which doesn't support exists queries
+        if (fieldPattern.startsWith("raw")) {
+            fieldPattern = TEXT_FIELD_NAME;
+        }
+
         // also sometimes test wildcard patterns
         if (randomBoolean()) {
-            if (randomBoolean()) {
+            if (randomBoolean() && !fieldPattern.equals("r") && !fieldPattern.equals("ra")) {
                 fieldPattern = fieldPattern + "*";
             }
         }
+
         return new ExistsQueryBuilder(fieldPattern);
     }