Omit zero-valued data from marshalled APIResponse

[johnmaguire]

What

Tags APIResponse.Data with omitzero and APIResponse.Errors with omitempty, so marshalling omits whichever side is unset. Adds round-trip tests for both the error-only and data-only cases.

Why

The API sends exactly one of data or errors - success responses have no errors key, and error responses have no data key. Go test servers marshalling an error-only APIResponse emitted a zero-valued data object alongside the errors. Its empty network.curve then failed NetworkCurve validation when the client decoded the response, turning the intended API errors into:

unexpected error during enrollment: error decoding JSON response: invalid network curve
body: {"data":{...},"errors":[{"code":"ERR_UNEXPECTED_HOSTNAME",...}]}

[nbrownus]

Nit: the 5-line comment on APIResponse could be one line, something like // The API sends exactly one of data or errors. The test comments restate the PR description and could go too.

[jasikpark]

Looks good: omitzero is the right tag for the generic Data T. Ready to approve once the test below is tightened.

One request before merge: TestEnroll's error case (client_test.go:173) gave a false pass on this exact bug. require.Errorf's second argument is a failure message, not an expected value, so it passed on the invalid network curve decode error. ErrorContains wouldn't catch it either, because the decode error echoes the body. Swapping to

require.EqualError(t, err, "unexpected error during enrollment: "+errorMsg)

fails on main and passes here, so it pins the fix end-to-end through the client.

— Caleb + Cache

[jasikpark]

Thanks — EqualError pins it. LGTM.

— Caleb + Cache