fix(fractional): support nested JSON Logic, boolean names, negative weight clamping, null key error

[aepfli]

Summary

Three related fixes to the fractional operator uncovered by new testbed scenarios (flagd-testbed PRs #341-#357).

1. Evaluate inner bucket elements as JSON Logic

Previously, the outer [name_expr, weight_expr] array was evaluated but its inner elements were passed as-is to fractional(). Now each inner element is evaluated through the JSON Logic evaluator, enabling nested expressions as variant names/weights and boolean literals as bucket names.

2. Clamp negative weights to zero

Negative weights are now clamped to 0 so the bucket participates in the list but receives no traffic — matching expected spec behavior.

3. Return error on null bucket key expression

When the first argument is a JSON Logic expression that evaluates to null or non-string, return an error so the flag engine falls back to defaultVariant.

All existing tests pass with zero regressions. New unit tests added for each fix.

[gemini-code-assist]

Code Review

This pull request enhances the fractional operator by allowing bucket names to be any JSON scalar (string, boolean, or number) and ensuring that nested JSON Logic within bucket definitions is evaluated. It also implements clamping for negative weights and refines the bucketing logic to handle zero-weight scenarios more gracefully. Review feedback recommends optimizing performance by avoiding an unnecessary clone of context data and refactoring the manual loop in the fractional function to use more idiomatic Rust iterators.