chore: handles offline mode, no initializers nor synchronizers

[tanderson-ld]

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

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Note

Medium Risk
Changes affect initialization and offline data source selection, which can alter client connectivity/status behavior. Persistence propagation tweaks also impact whether updates are written to persistent stores.

Overview
FDv2 now behaves cleanly in offline and no-source configurations. FDv2DataSystem builds an externalUpdatesOnly data source when config.offline is true, and FDv2DataSource explicitly marks the client VALID (with a log) when there are no initializers/synchronizers instead of trying to connect.

Persistence signaling is now consistently propagated. DataSourceSynchronizerAdapter includes shouldPersist when converting FDv1 init payloads into FDv2 ChangeSets and defaults FDv1 upsert conversions to persist, while WriteThroughStore simplifies/guards persistent-store calls using hasPersistence.

Adds a regression test to ensure an offline client using the FDv2 data system initializes successfully and reports VALID status.

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