add StatelessSession.getIdentifier() because repositories need it

this was already added to ORM SS in 6.6

hibernate-reactive-core/src/main/java/org/hibernate/reactive/mutiny/Mutiny.java

@@ -1877,6 +1877,17 @@ default Uni<Void> refresh(Object entity, LockModeType lockModeType) {
 		 */
 		<T> Uni<T> fetch(T association);
 
+		/**
+		 * Return the identifier value of the given entity, which may be detached.
+		 *
+		 * @param entity a persistent instance associated with this session
+		 *
+		 * @return the identifier
+		 *
+		 * @since 3.0
+		 */
+		Object getIdentifier(Object entity);
+
 		/**
 		 * Obtain a native SQL result set mapping defined via the annotation
 		 * {@link jakarta.persistence.SqlResultSetMapping}.

hibernate-reactive-core/src/main/java/org/hibernate/reactive/mutiny/impl/MutinyStatelessSessionImpl.java

@@ -193,6 +193,11 @@ public <T> Uni<T> fetch(T association) {
 		return uni( () -> delegate.reactiveFetch( association, false ) );
 	}
 
+	@Override
+	public Object getIdentifier(Object entity) {
+		return delegate.getIdentifier(entity);
+	}
+
 //	@Override
 //	public <T> ResultSetMapping<T> getResultSetMapping(Class<T> resultType, String mappingName) {
 //		return delegate.getResultSetMapping( resultType, mappingName );

hibernate-reactive-core/src/main/java/org/hibernate/reactive/session/ReactiveStatelessSession.java

@@ -71,4 +71,6 @@ public interface ReactiveStatelessSession extends ReactiveQueryProducer, Reactiv
 	boolean isOpen();
 
 	void close(CompletableFuture<Void> closing);
+
+	Object getIdentifier(Object entity);
 }

hibernate-reactive-core/src/main/java/org/hibernate/reactive/stage/Stage.java

@@ -1916,6 +1916,17 @@ default CompletionStage<Void> refresh(Object entity, LockModeType lockModeType)
 		 */
 		<T> CompletionStage<T> fetch(T association);
 
+		/**
+		 * Return the identifier value of the given entity, which may be detached.
+		 *
+		 * @param entity a persistent instance associated with this session
+		 *
+		 * @return the identifier
+		 *
+		 * @since 3.0
+		 */
+		Object getIdentifier(Object entity);
+
 		/**
 		 * Obtain a native SQL result set mapping defined via the annotation
 		 * {@link jakarta.persistence.SqlResultSetMapping}.

hibernate-reactive-core/src/main/java/org/hibernate/reactive/stage/impl/StageStatelessSessionImpl.java

@@ -132,6 +132,11 @@ public <T> CompletionStage<T> fetch(T association) {
 		return delegate.reactiveFetch( association, false );
 	}
 
+	@Override
+	public Object getIdentifier(Object entity) {
+		return delegate.getIdentifier(entity);
+	}
+
 	@Override
 	public <T> CompletionStage<T> withTransaction(Function<Stage.Transaction, CompletionStage<T>> work) {
 		return currentTransaction == null