feat(fractional): add nested fractional expression gherkin scenarios

[aepfli]

Summary

Adds gherkin test coverage for nested JSON Logic expressions as bucket variant names in the fractional operator — tracking open-feature/flagd-evaluator#30 and open-feature/flagd-evaluator#31.

New flag definitions

• fractional-nested-if-flag — variant name is an {"if": [...]} expression resolved from context
• fractional-nested-var-flag — variant name is a {"var": "..."} expression read from context

New scenarios

Both tagged @fractional @fractional-nested:

• Fractional operator with nested if expression as variant name
• Fractional operator with nested var expression as variant name

Transition support

The dedicated @fractional-nested tag lets providers opt out during their migration period:

-t 'not @fractional-nested'

Verification

All rows verified against the Rust flagd-evaluator gherkin test suite (worktree feat/nested-fractional).

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly expands the test suite for the fractional operator by introducing scenarios that validate its functionality when bucket variant names are derived from nested JSON Logic expressions. This ensures the evaluator correctly handles dynamic variant resolution, improving the robustness and reliability of flag targeting.

Highlights

• Enhanced Fractional Operator Testing: Added comprehensive Gherkin test coverage for the fractional operator to validate its behavior with nested JSON Logic expressions used as bucket variant names.
• New Flag Definitions: Introduced two new flag definitions, fractional-nested-if-flag and fractional-nested-var-flag, in flags/custom-ops.json to demonstrate and test dynamic variant name resolution.
• New Gherkin Scenarios: Created two new Gherkin scenarios, "Fractional operator with nested if expression as variant name" and "Fractional operator with nested var expression as variant name", to specifically test the new flag definitions.
• Transition Support: Implemented a dedicated @fractional-nested tag to allow providers to opt out of these new tests during their migration period.

Changelog

• flags/custom-ops.json
  • Added fractional-nested-if-flag definition, using an if expression for variant name.
  • Added fractional-nested-var-flag definition, using a var expression for variant name.
• gherkin/targeting.feature
  • Added "Fractional operator with nested if expression as variant name" scenario.
  • Added "Fractional operator with nested var expression as variant name" scenario.

Activity

• No explicit activity (comments, reviews, progress updates) has been recorded for this pull request yet.

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[gemini-code-assist]

Code Review

This pull request adds Gherkin test scenarios for nested JSON Logic expressions within the fractional operator, which is a valuable addition for ensuring evaluator correctness. The new flag definitions in flags/custom-ops.json and the corresponding scenarios in gherkin/targeting.feature are well-structured. I've suggested one improvement to enhance the test coverage for the fractional-nested-var-flag scenario by including cases where the resolved variant is invalid or missing, to ensure robust fallback behavior.