refactor: Modify the triggering method of the variable selector in the modification object subtree panel.

- Change the "onClick" event to "onMouseDown" event to address any potential issues related to mouse events, such as when using Http json editor to select structured data, the position offset after clicking causes the inability to select.

web/app/components/workflow/nodes/_base/components/variable/object-child-tree-panel/picker/field.tsx

@@ -41,7 +41,7 @@ const Field: FC<Props> = ({
       <Tooltip popupContent={t('app.structOutput.moreFillTip')} disabled={depth !== MAX_DEPTH + 1}>
         <div
           className={cn('flex items-center justify-between rounded-md pr-2', !readonly && 'hover:bg-state-base-hover', depth !== MAX_DEPTH + 1 && 'cursor-pointer')}
-          onClick={() => !readonly && onSelect?.([...valueSelector, name])}
+          onMouseDown={() => !readonly && onSelect?.([...valueSelector, name])}
         >
           <div className='flex grow items-stretch'>
             <TreeIndentLine depth={depth} />