fix: make XXE protection test locale-resilient (#2102)

- The `GenericJaxbIOTest.readOverridingNamespaceIsProtectedFromXXE` test
asserts on the English error message `"DOCTYPE is disallowed"`, which
fails on non-English JVM locales (e.g. `de_DE` produces `"DOCTYPE ist
nicht zulässig"`)
- Changed the assertion to match `"disallow-doctype-decl"` instead — the
XML parser feature name, which is locale-independent and always present
in the `SAXParseException` message

---------
Co-authored-by: Dolverin <dolverin@users.noreply.github.com>
Co-authored-by: Lukáš Petrovický <lukas@petrovicky.net>

Author: Dolverin

core/src/test/java/ai/timefold/solver/core/impl/io/jaxb/GenericJaxbIOTest.java

@@ -47,7 +47,7 @@ void readOverridingNamespaceIsProtectedFromXXE() {
         assertThatExceptionOfType(TimefoldXmlSerializationException.class)
                 .isThrownBy(() -> xmlIO.readOverridingNamespace(new StringReader(maliciousXml)))
                 .withRootCauseExactlyInstanceOf(SAXParseException.class)
-                .withStackTraceContaining("DOCTYPE is disallowed");
+                .withStackTraceContaining("disallow-doctype-decl");
     }
 
     @XmlRootElement