feat: Add FDv2 support to the test data source

[beekld]

Summary

The FDv1 TestData source does not work with the FDv2 data system, so tests that rely on it cannot run against FDv2. This teaches the same TestData to also act as an FDv2 synchronizer, so it can be passed to ConfigBuilder::data_source for FDv1 or to DataSystemBuilder::synchronizer for FDv2.

Under FDv2 it delivers a full payload of the current flags on first read, then a partial for each later update. The FDv2 data system is pull-based, so the source hands change sets to it rather than writing the store directly. As a result, updates propagate asynchronously, unlike FDv1's synchronous update().

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Note

Overview
TestData now works with the FDv2 data system in addition to the existing FDv1 data_source path. Wire it via DataSystemBuilder::synchronizer instead of ConfigBuilder::data_source.

The implementation adds an FDv2 synchronizer that returns a full change set on the first pull, registers an mpsc channel for live updates, and broadcasts partial change sets when flags/segments change through update, use_preconfigured_flag, or use_preconfigured_segment. FDv1 behavior (direct store upserts) is unchanged. Docs note that FDv2 updates are asynchronous.

ItemChange gains Clone so partial sets can be fan-out to multiple synchronizers. New client and test_data tests cover end-to-end FDv2 evaluation and full-then-partial synchronizer events.

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