Fix SCM config radio index mismatch when isApplicable filters descriptors

When an SCM plugin uses isApplicable() to restrict itself to specific
job types, the radio button indices in the UI no longer match the
unfiltered descriptor list used during form submission.

The fix modifies SCMS.parseSCM() to use SCM._for(target), which returns
the same filtered list used by the UI, ensuring the selected index
correctly maps to the intended SCM descriptor.

Author: LakshyaBagani

core/src/main/java/hudson/scm/SCMS.java

@@ -27,10 +27,13 @@
 import hudson.Extension;
 import hudson.model.AbstractProject;
 import hudson.model.Descriptor.FormException;
+import hudson.model.Job;
 import hudson.util.DescriptorList;
 import io.jenkins.servlet.ServletExceptionWrapper;
 import jakarta.servlet.ServletException;
 import java.util.List;
+import net.sf.json.JSONObject;
+import org.kohsuke.stapler.Stapler;
 import org.kohsuke.stapler.StaplerRequest;
 import org.kohsuke.stapler.StaplerRequest2;
 
@@ -56,14 +59,25 @@ public class SCMS {
      */
     @SuppressWarnings("deprecation")
     public static SCM parseSCM(StaplerRequest2 req, AbstractProject target) throws FormException, ServletException {
-        SCM scm = SCM.all().newInstanceFromRadioList(req.getSubmittedForm().getJSONObject("scm"));
+        List<SCMDescriptor<?>> applicableDescriptors = SCM._for((Job) target);
+        SCM scm = newInstanceFromRadioList(applicableDescriptors, req.getSubmittedForm().getJSONObject("scm"));
         if (scm == null) {
             scm = new NullSCM(); // JENKINS-36043 workaround for AbstractMultiBranchProject.submit
         }
         scm.getDescriptor().incrementGeneration();
         return scm;
     }
 
+    private static SCM newInstanceFromRadioList(List<SCMDescriptor<?>> descriptors, JSONObject config) throws FormException {
+        if (config.isNullObject())
+            return null;
+        int idx = config.getInt("value");
+        if (idx < 0 || idx >= descriptors.size()) {
+            throw new FormException("Invalid SCM index: " + idx, "scm");
+        }
+        return descriptors.get(idx).newInstance(Stapler.getCurrentRequest2(), config);
+    }
+
     /**
      * @deprecated use {@link #parseSCM(StaplerRequest2, AbstractProject)}
      */