Fix false 'Build Scheduled' notification when parent multibranch is disabled

When trying to trigger a build of a branch job in a disabled multibranch
project, the UI showed 'Build Scheduled' even though the build was not
actually scheduled (queue returned null, causing a 302 redirect).

Changes:
- Updated isBuildable() to check if parent ParameterizedJob is disabled
- Changed doBuild() to return 409 CONFLICT when schedule2() returns null
- Fixed JavaScript to only treat HTTP 201 as success (not 302 redirects)

Fixes issue where disabled multibranch parent allows branch builds to
appear scheduled when they are actually rejected.

Author: LakshyaBagani

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;
@@ -231,7 +232,7 @@ public final void doBuild(StaplerRequest2 req, StaplerResponse2 rsp, @QueryParam
             // TODO JENKINS-66105 use SC_SEE_OTHER if !ScheduleResult.created
             rsp.sendRedirect(SC_CREATED, req.getContextPath() + '/' + item.getUrl());
         } else {
-            rsp.sendRedirect(".");
+            throw HttpResponses.errorWithoutStack(SC_CONFLICT, asJob().getFullName() + " is not buildable");
         }
     }
 
@@ -563,7 +564,19 @@ default RunT createExecutable() throws IOException {
         }
 
         default boolean isBuildable() {
-            return !isDisabled() && !((Job) this).isHoldOffBuildUntilSave();
+            if (isDisabled() || ((Job) this).isHoldOffBuildUntilSave()) {
+                return false;
+            }
+            // Check if parent (e.g., multibranch project) is disabled
+            // If parent is disabled, child jobs (e.g., branch jobs) should not be buildable
+            ItemGroup<? extends Item> parentGroup = ((Job) this).getParent();
+            if (parentGroup instanceof Item) {
+                Item parent = (Item) parentGroup;
+                if (parent instanceof ParameterizedJob && ((ParameterizedJob) parent).isDisabled()) {
+                    return false;
+                }
+            }
+            return true;
         }
 
         @Extension

core/src/main/resources/lib/hudson/project/configurable/configurable.js

@@ -10,7 +10,7 @@
         method: "post",
         headers: crumb.wrap({}),
       }).then((rsp) => {
-        if (rsp.ok) {
+        if (rsp.status === 201) {
           notificationBar.show(success, notificationBar.SUCCESS);
         } else {
           notificationBar.show(failure, notificationBar.ERROR);