test($evaluators): add gherkin test coverage for $ref resolution edge cases from flagd#1875

[aepfli]

Background

flagd#1875 documents inconsistencies in $evaluators/$ref resolution across SDK implementations, including missing Rust support. The flagd-evaluator has a complete implementation (recursive resolution, circular reference detection, parse-time validation for missing refs), but these scenarios are not covered by the shared gherkin test suite.

Missing Test Coverage

1. Basic $ref resolution

A flag using a shared targeting rule from $evaluators should evaluate correctly.

2. Unresolved $ref (missing evaluator)

A $ref pointing to a non-existent evaluator name.
Expected: flag reports an error at state-update time (parse-time validation), not silently at evaluation time.

3. Nested $ref (evaluator referencing another evaluator)

An evaluator that itself contains a $ref.
Expected: resolved recursively.

4. Circular $ref detection

An evaluator that references itself (directly or transitively).
Expected: error reported cleanly, not an infinite loop.

What Needs to Be Done

1. Add flag definitions to the testbed covering these scenarios
2. Add @evaluators gherkin scenarios to targeting.feature
3. Add those scenarios to the evaluator's tests/gherkin_tests.rs

Related

• flagd#1875 — root bug report
• go-sdk-contrib#849 — tracking issue