自动屏蔽 placeholder OAuth provider

[yaffir]

• 新增 isValidOAuthProvider() 方法（过滤空值和 placeholder）
• 在 listOAuthProviders() 和 listMethods() 中应用过滤

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}