fix(acme): 修复 internal 载荷属性在 AOT 下被丢弃导致 finalize CSR 为空

[239573049]

Order.Payload.Csr 与 Account.Payload.OnlyReturnExisting 声明为 internal， System.Text.Json 源生成器只序列化 public 成员，二者被静默丢弃：

• finalize 请求缺少 csr 字段，Let's Encrypt 报 'asn1: syntax error: sequence truncated'
• onlyReturnExisting 丢失会破坏"仅返回已有账号"流程

改为 public 并加 [JsonInclude]。外层 Payload 类本身是 internal， 不会扩大公共 API 表面。

本地验证：真实 CSR 经签名后 csr 字段完整往返（DER 278B，tag 0x30），
onlyReturnExisting 正常序列化。

Motivation and Context

How Has This Been Tested?

Breaking Changes

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context