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 #18724

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

Author: atris

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

@@ -89,9 +89,13 @@ protected ExistsQueryBuilder doCreateTestQueryBuilder() {
         }
         // also sometimes test wildcard patterns
         if (randomBoolean()) {
-            if (randomBoolean()) {
-                fieldPattern = fieldPattern + "*";
-            }
+            fieldPattern = fieldPattern + "*";
+        }
+        // Avoid patterns that would match the derived field "raw.derived_keyword"
+        // This includes "raw", "raw*", "raw.xxx", "r*", "ra*", etc.
+        if (fieldPattern.equals("raw") || fieldPattern.startsWith("raw") || 
+            (fieldPattern.contains("*") && "raw.derived_keyword".startsWith(fieldPattern.replace("*", "")))) {
+            fieldPattern = TEXT_FIELD_NAME;
         }
         return new ExistsQueryBuilder(fieldPattern);
     }