fix: CountEnvelope json.Marshal

[zhoupingl]

fx: CountEnvelope json.marshal error.

[sweep-ai[bot]]

Sweep: PR Review

envelopes.go

The change ensures correct JSON formatting by adding a comma before the count object in the CountEnvelope's MarshalJSON method.

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[zhoupingl]

这是来自QQ邮箱的假期自动回复邮件。   您好，我最近正在休假中，无法亲自回复您的邮件。我将在假期结束后，尽快给您回复。

[fiatjaf]

Thank you!