In plain words: 75% coverage is the absolute bare minimum, not the final goal. The real goal of testing is to write asserts that fail when behavior breaks. To make tests maintainable, build all test data using one central factory. When an admin adds a new required field, you fix one recipe instead of manually updating 400 separate tests. Remember: tests cannot make real callouts—you must use HttpCalloutMock , and you should use StubProvider to mock class dependencies. Finally, never rely on existing org data; ban SeeAllData=true unless absolutely necessary.
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