ESQL: Fix skipping of generative tests (#132390) (#132436)

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.

(cherry picked from commit ccfbf49)

# Conflicts:
#	muted-tests.yml

Author: alex-spies

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

@@ -144,7 +144,7 @@ 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;
                 }
             }
@@ -155,13 +155,24 @@ 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")
     private EsqlQueryGenerator.QueryExecuted execute(String command, int depth) {
         try {