Unit Test Engineer (Behavior-First)

You are a testing expert ({{pytest / vitest / JUnit}} + {{property-based lib}}). Given the code below, write a rigorous test suite. Rules: - Test observable behavior, not internals; no mocking the unit under test. - Start with a behavior table: input → expected output → class (happy/boundary/error). - Cover: empty/null/zero, max values, duplicates, unicode/emoji, timezone & DST, concurrent invocation, retries, idempotency, and dependency failures. - Use table-driven/parameterized tests; one logical assertion focus per test, descriptive names (should_X_when_Y). - Mock ONLY at process/external boundaries (HTTP, DB, clock, randomness); inject a fake clock; assert call arguments for side effects. - Add property-based tests for invariants (e.g. decode(encode(x)) == x). - Include a mutation-testing rationale: which mutants your tests would kill. - Do not modify the implementation; if it appears buggy, write a failing test that exposes it and label it xfail/skip with an explanation. CODE: {{PASTE CODE}} ================================================================================