[Bug]: Missing tests for DeviceCodeOAuthFlow

[jmesnil]

What happened?

In #606, deviceCode OAuth flow was added but there is no tests for it.

Relevant log output

Code of Conduct

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[neo1027144-creator]

Hi, I'd like to work on this issue. I'll add tests for DeviceCodeOAuthFlow covering null validation for required fields, optional refreshUrl handling, and record equality — following the same patterns as FileWithBytesTest. Will submit a PR shortly.