feat: fractional enhancements

[leakonvalinka]

This PR

• allows bucketing weights summing up to MaxInt32 (=2,147,483,647)
• allows nesting of and within the fractional operator
• allows all primitives as bucket variants

Related Issues

#1493 and #1492

Notes

Follow-up Tasks

Remove the exclusion of @fractional-v2 and fractional-nested from RPC e2e tests once flagd supports these changes

How to test

new unit tests and gherkin (e2e) tests

[gemini-code-assist]

Summary of Changes

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This pull request significantly enhances the fractional targeting operator by expanding its capabilities to handle larger bucketing weights, support a wider range of variant types, and allow for more complex, nested expressions within its configuration. These changes improve the flexibility and robustness of feature flag rollouts, enabling more granular control and dynamic variant assignments.

Highlights

• Fractional Weight Capacity: Increased the maximum sum of bucketing weights to Math.MaxInt32 (2,147,483,647) by utilizing BigInt for calculations, preventing overflow issues with large integer weights.
• Nested Fractional Operators: Enabled the nesting of 'if' and 'var' expressions within the fractional operator for dynamic variant assignment and computed weights.
• Primitive Variant Support: Extended support for bucket variants to include all primitive types (string, number, boolean, and null), enhancing flexibility in flag configurations.
• Single-Bucket Fractional Lists: Allowed fractional rules to be configured with a single bucket, simplifying 100% traffic allocation scenarios.
• Weight Validation and Clamping: Introduced validation to ensure weights are integers and total weight does not exceed MaxInt32, and negative computed weights are now clamped to 0.

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[gemini-code-assist]

Code Review

This pull request significantly enhances the fractional targeting rule in flagd-core. Key improvements include expanded variant type support (now allowing strings, numbers, booleans, and null), improved weight handling with validation for integer weights, clamping of negative weights to zero, and a check to prevent total weight overflow using Math.MaxInt32. The bucketing calculation now uses BigInt for enhanced precision with large integer weights, enabling sub-percent granularity. Additionally, the rule now supports single-bucket configurations. Extensive new test cases have been added to cover these new capabilities and edge cases. E2E tests have been updated to temporarily exclude new fractional features, indicating ongoing development. The flagd-core test harness subproject has also been updated. There is no feedback to provide.