fix: API calls throwing internal server errors

[deadlyjack]

No description provided.

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge; the request headers are compatible with the existing fetch wrapper and established API request conventions.

Summary

• Updates paid-plugin order requests across direct, dependency, and plugin-detail installation flows.
• Updates the paid-plugin refund request.
• Updates sponsor purchase registration.

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