fix(http): preserve Agnes error response bodies

[jomeswang]

Summary

• read Agnes HTTP bodies as text first so non-JSON and empty error responses are preserved
• parse JSON when possible and sanitize secrets before exposing non-2xx details
• avoid misclassifying HTTP errors as retryable network failures

Validation

• npm test (34 tests passed)
• real Agnes API: 1080p/385-frame request returned the concrete max_num_frames: 241 error through the CLI

No credentials are persisted or included in this PR.