fix: handle json.Unmarshal error in GetTestResult

[mugiwaraluffy56]

GetTestResult in microcks_client.go called json.Unmarshal but ignored the returned error. every other unmarshal call in the same file checks it, this one didn't.

the problem is that if the api returns something unexpected (an html error page, a rate-limit response, or a not-yet-ready response during startup), json.Unmarshal fails silently and result stays as a zero-value TestResultSummary with Success=false and InProgress=false. the polling loop in cmd/test.go sees inProgress=false, exits immediately, and the user gets a failure with no useful message.

the fix is straightforward: check the error and return it so the caller gets a real error instead of a silent bad result.

also removed the redundant []byte(body) cast since io.ReadAll already returns []byte.