Fix resource leaks in core file processing classes

- Add try-with-resources for FileInputStream in ZUGFeRDVisualizer
- Fix unclosed InputStream in ZUGFeRDInvoiceImporter with try-with-resources
- Add explicit close() call for XMLWriter in ZUGFeRDValidator
- Properly close DataInputStream in ZUGFeRDExporterFromPDFA
- Fix resource management in Main.convertInputStreamToString

validator/src/main/java/org/mustangproject/validator/ZUGFeRDValidator.java

@@ -301,6 +301,7 @@ public class ZUGFeRDValidator {
 		XMLWriter writer = new XMLWriter(sw, format);
 		try {
 			writer.write(document);
+			writer.close();
 		} catch (Exception e) {
 			LOGGER.error(e.getMessage());
 		}