feat: Evaluate flags that depend on other flags - #343
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Adds support for segments conditioned on another flag's result, via a `$.flags.<feature name>` condition property, as the evaluation half of dependent flags. Segment conditions are already JSONPath, so a dependency needs no new operator or condition type. What it does need is for the flag to be resolved by the time the condition reads it, and evaluation resolved every segment before any flag. Rather than establish up front which segments depend on which flags, `$.flags` is a mapping that resolves a flag when a condition first reads it. Resolving a flag evaluates the segments overriding it, which in turn resolves whatever flags their conditions read, memoised throughout. The existing single pass is otherwise untouched: a context whose segments read no flag never enters the resolver, and pays only for one empty mapping. Resolving on read rather than in advance means there is no need to recognise a dependency in a condition property, and so no need to reimplement enough of the JSONPath grammar to tell that `$.flags.a['enabled']` and `$.flags.a.enabled` are the same query. Every spelling works because resolution is triggered by the read itself. It also costs only what is read: rule short-circuiting means a condition that is never evaluated resolves nothing. Measured against main across the benchmark contexts, interleaved to cancel drift: +3.4%, from the mapping and a shallow copy of the context to hold it. Establishing up front that an environment has no dependencies would remove that residual entirely, but needs a field on the context to carry it. A flag caught in a dependency cycle is left unresolved rather than raising, so a cycle reaching the engine degrades to a non-matching condition instead of breaking evaluation for the whole context. Such a flag serves its environment default and reports `ERROR; code=CIRCULAR_DEPENDENCY`, so that a flag which could not be resolved is distinguishable from one that was never gated. Only flags in the cycle are reported that way; a flag merely depending on one resolves normally. Cycles are still expected to be rejected where dependencies are written. Behaviour is covered by test cases in Flagsmith/engine-test-data#59 rather than by unit tests here, so that every engine is held to it.
REVERT BEFORE MERGE. Dependent flags behaviour is covered by test cases in Flagsmith/engine-test-data#59 rather than by unit tests here, so that every engine is held to it. Those cases aren't in a release yet, so without this the new code is exercised by nothing and CI's 100% coverage gate fails. Once #59 is merged and tagged, this goes back to a semver tag, which Renovate now tracks as of #337.
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Contributes to Flagsmith/flagsmith#8394
Implements Flagsmith/engine-test-data#59