StatusCodeMap.FromMxStatus checked `success != 0` to determine success, but the
mxaccessgw proto contract explicitly documents that `success` is not a boolean and
that clients must branch on `category` (MX_STATUS_CATEGORY_OK), not on `success`
alone. Replace the raw field check with `status.IsSuccess()` from
MxStatusProxyExtensions, which requires both `success != 0` AND `category == Ok`.
A worker reporting success=1 with a non-OK category was previously misreported as
Good. Updated StatusCodeMapTests with a regression case covering the inverted scenario.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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