feat: Extract Signal interface for simple computed signals (#22166)

* Extract Signal interface for simple computed signals

Fixes #22121

* Fix javadoc reference

* Remove unused imports

Author: Legioth

flow-server/src/test/java/com/vaadin/flow/server/VaadinServiceTest.java

@@ -19,7 +19,6 @@
 import jakarta.servlet.ServletException;
 import jakarta.servlet.http.HttpSessionBindingEvent;
 
-import java.lang.reflect.Field;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
@@ -33,7 +32,6 @@
 import java.util.concurrent.Executor;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
-import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.concurrent.atomic.AtomicReference;
@@ -47,8 +45,6 @@
 import org.mockito.MockedStatic;
 import org.mockito.Mockito;
 
-import com.vaadin.experimental.DisabledFeatureException;
-import com.vaadin.experimental.FeatureFlags;
 import com.vaadin.flow.component.Component;
 import com.vaadin.flow.component.DetachEvent;
 import com.vaadin.flow.component.Tag;
@@ -74,12 +70,7 @@
 import com.vaadin.flow.server.communication.WebComponentProvider;
 import com.vaadin.flow.server.menu.AvailableViewInfo;
 import com.vaadin.flow.server.startup.ApplicationRouteRegistry;
-import com.vaadin.signals.ListSignal;
-import com.vaadin.signals.Signal;
-import com.vaadin.signals.SignalEnvironment;
-import com.vaadin.tests.util.AlwaysLockedVaadinSession;
 import com.vaadin.tests.util.MockDeploymentConfiguration;
-import com.vaadin.tests.util.MockUI;
 
 import static org.hamcrest.CoreMatchers.containsString;
 import static org.hamcrest.MatcherAssert.assertThat;