refactor: read the flag once per evaluation, in one place

[abelonogov-ld]

The ten variation methods each spelled out the same call to the hook runner around the same evaluation, and left the flag read to the evaluation itself. They now collapse onto a helper that does the read, so one place expresses the order of the read, the hooks and the evaluation, and what an evaluation is about to return can be described without reading the store a second time.

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Overview
Refactors flag evaluation so all typed *Variation APIs go through a new evaluateWithHooks helper instead of duplicating hookRunner.withEvaluation calls.

Evaluation now snapshots the evaluation context and flag (getNonDeletedFlag) once before hooks run, then passes those into variationDetailInternal. Prerequisite walks reuse the same context and fetch each prereq flag at that layer instead of re-reading context inside the evaluator.

Intent: one ordered pipeline (read → hooks → evaluate) so a concurrent identify() cannot swap context mid-call and attribute the wrong context on evaluation events.

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