Add isBuildable to BuildableItem interface

When disabling a Multibranch project the branch/PR jobs still show the
build button in the sidepanel and in the folder view the Build button
column allows to start a build.

By adding the isBuildable to the BuildableItem interface we can now
detect in Jobs if the parent is disabled. This will then hide the build
button in the sidepanel and the column in the view.

fixes jenkinsci#16678

Author: mawinter69

core/src/main/java/hudson/model/BuildableItem.java

@@ -57,4 +57,8 @@ default boolean scheduleBuild(int quietPeriod) {
     }
 
     boolean scheduleBuild(int quietPeriod, Cause c);
+
+    default boolean isBuildable() {
+        return true;
+    };
 }

core/src/main/java/jenkins/model/ParameterizedJobMixIn.java

@@ -40,6 +40,7 @@
 import hudson.model.Cause;
 import hudson.model.CauseAction;
 import hudson.model.Item;
+import hudson.model.ItemGroup;
 import hudson.model.Items;
 import hudson.model.Job;
 import hudson.model.ParameterDefinition;
@@ -562,8 +563,19 @@ default RunT createExecutable() throws IOException {
             return null;
         }
 
+        @Override
         default boolean isBuildable() {
-            return !isDisabled() && !((Job) this).isHoldOffBuildUntilSave();
+            if (isDisabled() || ((Job) this).isHoldOffBuildUntilSave()) {
+                return false;
+            }
+            // If parent is disabled, child jobs (e.g., branch jobs) should not be buildable
+            ItemGroup<? extends Item> parentGroup = ((Job) this).getParent();
+            if (parentGroup instanceof BuildableItem bi) {
+                if (!bi.isBuildable()) {
+                    return false;
+                }
+            }
+            return true;
         }
 
         @Extension