Remove assign policy project mutation

- it is no longer needed as `createSecurityPolicyProject` assigns
  the new project already
- create additional tests

ee/app/assets/javascripts/threat_monitoring/components/policy_editor/scan_execution_policy/lib/utils.js

-import assignPolicyProject from 'ee/threat_monitoring/graphql/mutations/assign_policy_project.mutation.graphql';
 import createPolicyProject from 'ee/threat_monitoring/graphql/mutations/create_policy_project.mutation.graphql';
 import createScanExecutionPolicy from 'ee/threat_monitoring/graphql/mutations/create_scan_execution_policy.mutation.graphql';
 import { gqClient } from 'ee/threat_monitoring/utils';
@@ -23,7 +22,7 @@ const checkForErrors = ({ errors }) => {
 const assignSecurityPolicyProject = async (projectPath) => {
   const {
     data: {
-      securityPolicyProjectCreate: { project, errors: createErrors },
+      securityPolicyProjectCreate: { project, errors },
     },
   } = await gqClient.mutate({
     mutation: createPolicyProject,
@@ -32,21 +31,7 @@ const assignSecurityPolicyProject = async (projectPath) => {
     },
   });
 
-  checkForErrors({ errors: createErrors });
-
-  const {
-    data: {
-      securityPolicyProjectAssign: { errors: assignErrors },
-    },
-  } = await gqClient.mutate({
-    mutation: assignPolicyProject,
-    variables: {
-      projectPath,
-      id: project.id,
-    },
-  });
-
-  return { ...project, errors: assignErrors };
+  return { ...project, errors };
 };
 
 /**

ee/app/assets/javascripts/threat_monitoring/graphql/mutations/assign_policy_project.mutation.graphql

-mutation assignPolicyProject($projectPath: ID!, $id: ProjectID!) {
-  securityPolicyProjectAssign(input: { projectPath: $projectPath, securityPolicyProjectId: $id }) {
-    errors
-  }
-}

ee/spec/frontend/threat_monitoring/components/policy_editor/scan_execution_policy/lib/utils_spec.js

 import { savePolicy } from 'ee/threat_monitoring/components/policy_editor/scan_execution_policy/lib/utils';
 import { DEFAULT_ASSIGNED_POLICY_PROJECT } from 'ee/threat_monitoring/constants';
-import assignPolicyProject from 'ee/threat_monitoring/graphql/mutations/assign_policy_project.mutation.graphql';
 import createPolicyProject from 'ee/threat_monitoring/graphql/mutations/create_policy_project.mutation.graphql';
 import createScanExecutionPolicy from 'ee/threat_monitoring/graphql/mutations/create_scan_execution_policy.mutation.graphql';
 import { gqClient } from 'ee/threat_monitoring/utils';
@@ -31,12 +30,6 @@ const mockApolloResponses = (shouldReject) => {
           },
         },
       });
-    } else if (mutation === assignPolicyProject) {
-      return Promise.resolve({
-        data: {
-          securityPolicyProjectAssign: { errors: [] },
-        },
-      });
     } else if (mutation === createScanExecutionPolicy) {
       return Promise.resolve({
         data: {

ee/spec/helpers/projects/security/policies_helper_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Projects::Security::PoliciesHelper do
+  describe '#assigned_policy_project' do
+    let(:project) { create(:project) }
+
+    context 'when a project does have a security policy project' do
+      let(:policy_management_project) { create(:project) }
+      let!(:policy_configuration) { create(:security_orchestration_policy_configuration, security_policy_management_project: policy_management_project, project: project) }
+
+      subject { helper.assigned_policy_project(project) }
+
+      it {
+        is_expected.to include({
+          id: policy_management_project.to_global_id.to_s,
+          name: policy_management_project.name,
+          full_path: policy_management_project.full_path,
+          branch: policy_management_project.default_branch_or_main
+        })
+      }
+    end
+
+    context 'when a project does not have a security policy project' do
+      subject { helper.assigned_policy_project(project) }
+
+      it {
+        is_expected.to be_nil
+      }
+    end
+  end
+end