principle-test-behavior-not-implementation

# Test Behavior, Not Implementation A test calls the code as its users do and asserts the exact result they see. Rewrite or delete any test that would still pass if every imported function returned `undefined` or a wrong value. These shapes fail that check: - Loose or no assertion: `toBeDefined`, `toBeTruthy`, `not.toThrow`, `toBeGreaterThan(0)`. - Call or absence only: `toHaveBeenCalled`, `not.toHaveBeenCalled`, `toBeUndefined`, `toEqual([])`. - Expected value computed by the code under test: `expect(f(a)).toBe(f(a))`. - Restated constant, config default, table row, or prompt string. - Assertions on data the test built, with no call to the subject. Fix: call the subject in the test body with one concrete input and assert the literal output or effect. For a mock, assert the payload it received. For an absence, also assert the presence on another input. For a constant, test the mechanism that reads it with one input instead of restating the value. Keep relation checks across table rows (a key in two tables, a parent that exists) and `*.test-d.ts` type checks.