Address Gemini review: guard primaryConstructor against empty ctors

If Class.getConstructors() returns empty (e.g. all public constructors
removed in a future Spark version), throw an IllegalStateException with
the class name instead of letting ctors[0] surface as a cryptic
ArrayIndexOutOfBoundsException during static initialization.

Author: tkaymak

runners/spark/src/main/java/org/apache/beam/runners/spark/structuredstreaming/translation/helpers/EncoderFactory.java

@@ -46,6 +46,9 @@ public class EncoderFactory {
   @SuppressWarnings("unchecked")
   private static <T> Constructor<T> primaryConstructor(Class<T> cls) {
     Constructor<?>[] ctors = cls.getConstructors();
+    if (ctors.length == 0) {
+      throw new IllegalStateException("No public constructors found for " + cls.getName());
+    }
     Constructor<?> widest = ctors[0];
     for (int i = 1; i < ctors.length; i++) {
       if (ctors[i].getParameterCount() > widest.getParameterCount()) {