test(operators): add gherkin test coverage for custom operator error returns from flagd#1874

[aepfli]

Background

flagd#1874 documents that custom operators must return null (not false) on error to trigger the default variant fallback in JSONLogic. The flagd-evaluator already implements the correct behavior for all operators, but error-path behavior is not covered by the shared gherkin test suite.

Missing Test Coverage

sem_ver — invalid version string

Rule: {"sem_ver": [{"var": "version"}, "=", "not-a-version"]}
Expected: returns null → default variant fallback.

sem_ver — unknown operator

Rule: {"sem_ver": [{"var": "version"}, "!=", "1.0.0"]} with an unknown comparator.
Expected: returns null → default variant fallback.

fractional — no bucket matched

When weights are misconfigured and no bucket is matched.
Expected: returns null → default variant fallback (not empty string or exception).

fractional — invalid input

Empty bucket list, missing bucket key, etc.
Expected: returns null → default variant fallback.

What Needs to Be Done

1. Add flag definitions to the testbed for error-path scenarios
2. Add @operator-errors gherkin scenarios covering each error path
3. Add those scenarios to the evaluator's tests/gherkin_tests.rs

Related

• flagd#1874 — root bug report
• go-sdk-contrib#849 — tracking issue