Add test for issue #315 using GeneratorInitializer to write custom XML declaration encoding

[Copilot]

Issue #315 reports that the XML declaration encoding is hard-coded to UTF-8, with no supported mechanism to emit a different encoding (e.g. ISO-8859-1). The GeneratorInitializer extension point added in jackson-databind 3.2 (databind#5860) provides a clean solution: the initializer receives the freshly-created ToXmlGenerator before any content is written, allowing direct access to the underlying XMLStreamWriter to write a custom <?xml?> declaration.

Changes

• New test GeneratorInitializer315Test in src/test/java/…/xml/ser/ demonstrating the workaround:
  • Registers a GeneratorInitializer via XmlMapper.builder().generatorInitializer(...)
  • Casts JsonGenerator → ToXmlGenerator, retrieves XMLStreamWriter via getStaxWriter()
  • Calls sw.writeStartDocument("ISO-8859-1", "1.0") before Jackson writes any content
  • Asserts the serialized output contains the custom encoding in the XML declaration

GeneratorInitializer initializer = (config, gen) -> {
    XMLStreamWriter sw = ((ToXmlGenerator) gen).getStaxWriter();
    sw.writeStartDocument("ISO-8859-1", "1.0");
};
XmlMapper mapper = XmlMapper.builder()
        .generatorInitializer(initializer)
        .build();
String xml = mapper.writeValueAsString(new StringBean("test"));
assertTrue(xml.contains("encoding='ISO-8859-1'"));

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• central.sonatype.com
  • Triggering command: /usr/lib/jvm/temurin-17-jdk-amd64/bin/java /usr/lib/jvm/temurin-17-jdk-amd64/bin/java --enable-native-access=ALL-UNNAMED -classpath /usr/share/apache-maven-3.9.14/boot/plexus-classworlds-2.9.0.jar -Dclassworlds.conf=/usr/share/apache-maven-3.9.14/bin/m2.conf -Dmaven.home=/usr/share/apache-maven-3.9.14 -Dlibrary.jansi.path=/usr/share/apache-maven-3.9.14/lib/jansi-native -Dmaven.multiModuleProjectDirectory=/home/REDACTED/work/jackson-dataformat-xml/jackson-dataformat-xml org.codehaus.plexus.classworlds.launcher.Launcher dependency:resolve -q (dns block)
  • Triggering command: /usr/lib/jvm/temurin-17-jdk-amd64/bin/java /usr/lib/jvm/temurin-17-jdk-amd64/bin/java --enable-native-access=ALL-UNNAMED -classpath /usr/share/apache-maven-3.9.14/boot/plexus-classworlds-2.9.0.jar -Dclassworlds.conf=/usr/share/apache-maven-3.9.14/bin/m2.conf -Dmaven.home=/usr/share/apache-maven-3.9.14 -Dlibrary.jansi.path=/usr/share/apache-maven-3.9.14/lib/jansi-native -Dmaven.multiModuleProjectDirectory=/home/REDACTED/work/jackson-dataformat-xml/jackson-dataformat-xml org.codehaus.plexus.classworlds.launcher.Launcher help:effective-pom -q (dns block)
  • Triggering command: /usr/lib/jvm/temurin-17-jdk-amd64/bin/java /usr/lib/jvm/temurin-17-jdk-amd64/bin/java --enable-native-access=ALL-UNNAMED -classpath /usr/share/apache-maven-3.9.14/boot/plexus-classworlds-2.9.0.jar -Dclassworlds.conf=/usr/share/apache-maven-3.9.14/bin/m2.conf -Dmaven.home=/usr/share/apache-maven-3.9.14 -Dlibrary.jansi.path=/usr/share/apache-maven-3.9.14/lib/jansi-native -Dmaven.multiModuleProjectDirectory=/home/REDACTED/work/jackson-dataformat-xml/jackson-dataformat-xml org.codehaus.plexus.classworlds.launcher.Launcher dependency:list -q (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

[pjfanning]

added a test with ae7e650