Fix XML Namespace Handling in serializeExceptionToXml

In the commit 1b59089,
we inadvertently introduced an issue in the
serializeExceptionToXml method where the XML namespace was
set in a manner not compliant with XML standards,
causing issues in XML parsing.
Additionally, the method did not utilize the currentDatabindContext
for writing the error-path, which could lead to incorrect
serialization of error paths in certain contexts.

The fix involves correctly setting the default XML namespace for
the errors element. Additionally, the method now properly utilizes
the currentDatabindContext for encoding error paths.

JIRA: NETCONF-1130
Change-Id: Ia7900bff2e63d23213896b5e0c96a514ace92873
Signed-off-by: Yaroslav Lastivka <yaroslav.lastivka@pantheon.tech>

restconf/restconf-nb/src/main/java/org/opendaylight/restconf/nb/rfc8040/jersey/providers/errors/RestconfDocumentedExceptionMapper.java

@@ -189,9 +189,11 @@ private static String serializeExceptionToXml(final RestconfDocumentedException
         try (var outputStream = new ByteArrayOutputStream()) {
             final var xmlWriter = XML_OUTPUT_FACTORY.createXMLStreamWriter(outputStream,
                 StandardCharsets.UTF_8.name());
+            final var currentDatabindContext = exception.modelContext() != null
+                ? DatabindContext.ofModel(exception.modelContext()) : databindProvider.currentDatabind();
             xmlWriter.writeStartDocument();
             xmlWriter.writeStartElement(Errors.QNAME.getLocalName());
-            xmlWriter.writeNamespace("xmlns", Errors.QNAME.getNamespace().toString());
+            xmlWriter.writeDefaultNamespace(Errors.QNAME.getNamespace().toString());
             if (exception.getErrors() != null && !exception.getErrors().isEmpty()) {
                 for (final var error : exception.getErrors()) {
                     xmlWriter.writeStartElement(Error.QNAME.getLocalName());
@@ -202,7 +204,7 @@ private static String serializeExceptionToXml(final RestconfDocumentedException
 
                     if (error.getErrorPath() != null) {
                         xmlWriter.writeStartElement(ERROR_PATH_QNAME.getLocalName());
-                        databindProvider.currentDatabind().xmlCodecs().instanceIdentifierCodec()
+                        currentDatabindContext.xmlCodecs().instanceIdentifierCodec()
                             .writeValue(xmlWriter, error.getErrorPath());
                         xmlWriter.writeEndElement();
                     }