Coverage measures which lines ran, not which behaviours are protected. Those are different questions and only one of them matters at three in the morning.

Where the bugs actually live

In the incidents I have reviewed, almost none were a method returning the wrong arithmetic. They were boundaries: a serialisation change, a timezone, a nullable column that became non-nullable, a retry that was not idempotent.

• Test the contract at the edge of your service, not the getters behind it.
• Pin serialisation with a real payload from production, not a builder.
• Give every retry an idempotency test, because that is where duplicate charges come from.

Delete the tests that only assert the mock was called. They cost maintenance and catch nothing.