fix: add @nonnull value type annotations on collection-based signals (#23557)

In a @NullMarked package, type parameters are implicitly @nullable,
making peek() return @nullable for ListSignal, SharedListSignal, and
SharedMapSignal even though their collection values are never null.
Add @nonnull to the type arguments to correctly express this contract.

Author: Artur-

flow-server/src/main/java/com/vaadin/flow/signals/local/ListSignal.java

@@ -20,6 +20,8 @@
 import java.util.List;
 import java.util.Objects;
 
+import org.jspecify.annotations.NonNull;
+
 /**
  * A local list signal that holds a list of writable signals, enabling per-entry
  * reactivity.
@@ -36,7 +38,8 @@
  * @param <T>
  *            the element type
  */
-public class ListSignal<T> extends AbstractLocalSignal<List<ValueSignal<T>>> {
+public class ListSignal<T>
+        extends AbstractLocalSignal<@NonNull List<ValueSignal<T>>> {
 
     /**
      * Creates a new empty list signal.

flow-server/src/main/java/com/vaadin/flow/signals/shared/SharedListSignal.java

@@ -21,6 +21,7 @@
 import java.util.Objects;
 import java.util.stream.Collectors;
 
+import org.jspecify.annotations.NonNull;
 import org.jspecify.annotations.Nullable;
 import org.slf4j.LoggerFactory;
 
@@ -46,7 +47,7 @@
  *            the element type
  */
 public class SharedListSignal<T>
-        extends AbstractSignal<List<SharedValueSignal<T>>> {
+        extends AbstractSignal<@NonNull List<SharedValueSignal<T>>> {
 
     /**
      * A list insertion position before and/or after the referenced entries. If

flow-server/src/main/java/com/vaadin/flow/signals/shared/SharedMapSignal.java

@@ -23,6 +23,7 @@
 import java.util.Objects;
 import java.util.stream.Collectors;
 
+import org.jspecify.annotations.NonNull;
 import org.jspecify.annotations.Nullable;
 import org.slf4j.LoggerFactory;
 
@@ -49,7 +50,7 @@
  *            the element type
  */
 public class SharedMapSignal<T>
-        extends AbstractSignal<Map<String, SharedValueSignal<T>>> {
+        extends AbstractSignal<@NonNull Map<String, SharedValueSignal<T>>> {
 
     private Class<T> elementType;