失败的 c.req.formData() 会导致所有其他 body 方法抛出异常

**1.** The first call, `c.req.formData()` on a `Content-Type: application/json` request. `bodyCache` is still empty, and line 243 stores `raw.formData()`'s promise into it unconditionally — including when that promise is about to reject.