ESQL: Fix skipping of generative tests (elastic#132390)

The error message can sometimes be split into multiple lines with
backslashes marking the line boundaries. Let's make our skips work even
in case of overlong lines.

Author: alex-spies

muted-tests.yml

@@ -503,9 +503,6 @@ tests:
 - class: org.elasticsearch.xpack.sql.qa.mixed_node.SqlCompatIT
   method: testNullsOrderWithMissingOrderSupportQueryingNewNode
   issue: https://github.com/elastic/elasticsearch/issues/132249
-- class: org.elasticsearch.xpack.esql.qa.multi_node.GenerativeIT
-  method: test
-  issue: https://github.com/elastic/elasticsearch/issues/132273
 - class: org.elasticsearch.common.logging.JULBridgeTests
   method: testThrowable
   issue: https://github.com/elastic/elasticsearch/issues/132280

x-pack/plugin/esql/qa/server/src/main/java/org/elasticsearch/xpack/esql/qa/rest/generative/GenerativeRestTest.java

@@ -160,7 +160,7 @@ private static CommandGenerator.ValidationResult checkResults(
         );
         if (outputValidation.success() == false) {
             for (Pattern allowedError : ALLOWED_ERROR_PATTERNS) {
-                if (allowedError.matcher(outputValidation.errorMessage()).matches()) {
+                if (isAllowedError(outputValidation.errorMessage(), allowedError)) {
                     return outputValidation;
                 }
             }
@@ -171,13 +171,24 @@ private static CommandGenerator.ValidationResult checkResults(
 
     private void checkException(EsqlQueryGenerator.QueryExecuted query) {
         for (Pattern allowedError : ALLOWED_ERROR_PATTERNS) {
-            if (allowedError.matcher(query.exception().getMessage()).matches()) {
+            if (isAllowedError(query.exception().getMessage(), allowedError)) {
                 return;
             }
         }
         fail("query: " + query.query() + "\nexception: " + query.exception().getMessage());
     }
 
+    /**
+     * Long lines in exceptions can be split across several lines. When a newline is inserted, the end of the current line and the beginning
+     * of the new line are marked with a backslash {@code \}; the new line will also have whitespace before the backslash for aligning.
+     */
+    private static final Pattern ERROR_MESSAGE_LINE_BREAK = Pattern.compile("\\\\\n\\s*\\\\");
+
+    private static boolean isAllowedError(String errorMessage, Pattern allowedPattern) {
+        String errorWithoutLineBreaks = ERROR_MESSAGE_LINE_BREAK.matcher(errorMessage).replaceAll("");
+        return allowedPattern.matcher(errorWithoutLineBreaks).matches();
+    }
+
     @SuppressWarnings("unchecked")
     public static EsqlQueryGenerator.QueryExecuted execute(String command, int depth, @Nullable ProfileLogger profileLogger) {
         try {