Generates negative / error-path test cases that mirror happy-path tests - for each happy-path test, produces companions exercising input validation rejection, missing required fields, type mismatches, authorization failures, rate-limit errors, and adversarial payloads from the malicious-payload-bank. Emits cases as parameterized tests in the project's runner format. Use when a feature has happy-path coverage but the rejection / error / unauthorized paths are untested.