Tests have nothing to do with it. You can have 100% coverage on your API and it can still not be "quality". The best APIs I've seen have been "API First" design.
When talking about API "quality" I mean "does what it says on the tin, reliably".
Testing means you catch regressions.
You seem to be talking about API design, which is honestly a minor concern compared to whether it works reliably.
Have you ever tried to use the Facebook API? It breaks randomly such as endpoints suddenly return 404's, and there is no followup to bug reports.
That is what I'm talking about when I say "quality".
A "perfectly designed" API that randomly breaks because someone made a change and the lack of tests means its an end-user who discovers it is worse than terrible IMO.
Testing means you catch regressions.
You seem to be talking about API design, which is honestly a minor concern compared to whether it works reliably.
Have you ever tried to use the Facebook API? It breaks randomly such as endpoints suddenly return 404's, and there is no followup to bug reports.
That is what I'm talking about when I say "quality".
A "perfectly designed" API that randomly breaks because someone made a change and the lack of tests means its an end-user who discovers it is worse than terrible IMO.