Set Default Trace Context Status To OK

### Description

[Cocoa](https://github.com/getsentry/sentry-cocoa/issues/6230) and [Java](https://github.com/getsentry/sentry-java/issues/4769) set the default trace context status to unknown, which confuses users. Please double-check if this is also the case on Dart, and if yes, please change it to OK, which also aligns with span first.