fix: [Platform:StackManagement:Kibana:Spaces] Assign roles to a space flyout announced incorrectly (#218576)

Closes: #218351

**Description**
When user clicks on assign new roles to a space button, the resulting
flyout gets announced incorrectly as "you are in a modal dialog..."
which doesn't give any context to non-sighted user.

**Changes made:**
1. added 'aria-labelledby' attribute

**Screen:**
<img width="1323" alt="image"
src="https://github.com/user-attachments/assets/63ffdcca-2139-4302-9dce-ded3a4d3b9a2"
/>

x-pack/platform/plugins/shared/spaces/public/management/edit_space/edit_space_roles_tab.tsx

@@ -106,6 +106,7 @@ export const EditSpaceAssignedRolesTab: FC<Props> = ({ space, features, isReadOn
           size: 'm',
           maxWidth: true,
           maskProps: { headerZindexLocation: 'below' },
+          'aria-labelledby': 'space-assign-role-privilege-form-title',
         }
       );
     },

x-pack/platform/plugins/shared/spaces/public/management/edit_space/roles/component/space_assign_role_privilege_form.tsx

@@ -663,7 +663,7 @@ export const PrivilegesRolesForm: FC<PrivilegesRolesFormProps> = (props) => {
     <React.Fragment>
       <EuiFlyoutHeader hasBorder>
         <EuiTitle size="m">
-          <h2>
+          <h2 id="space-assign-role-privilege-form-title">
             {isEditOperation.current
               ? i18n.translate('xpack.spaces.management.spaceDetails.roles.assignRoleButton', {
                   defaultMessage: 'Edit role privileges for space',