feat: Honor the service's FDv1 fallback directive in the client

[beekld]

Summary

Honors the service's directive to move the client off FDv2 back to FDv1, and provides an FDv1 source to fall back to.

• Adds an FDv1 adapter synchronizer that wraps an existing FDv1 client source so it can act as an FDv2 synchronizer.
• On the fallback directive (a response header or a goodbye), applies any payload that arrived with it, then rotates from the FDv2 sources to the FDv1 fallback.
• Retries FDv2 after the directive's TTL expires.

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Note

Overview
Adds FDv1 fallback when the service tells the client to leave FDv2: a new FDv1AdapterSynchronizer exposes an existing FDv1 IDataSource as an FDv2 synchronizer (Init/Upsert/status → changesets and FDv2SourceResult).

FDv2DataSource now reacts to fdv1_fallback on initializer and synchronizer results: it still applies any accompanying payload, switches the synchronizer tier via SourceManager, starts the FDv1 fallback when configured, and schedules an FDv2 retry after the directive TTL (cancellable on close). If no FDv1 tier is configured, it reports interrupted with a clear message instead of staying on FDv2.

Unit tests cover the adapter lifecycle/conversions and orchestration (apply-before-fallback, FDv1 tier start, disconnect without fallback, TTL retry).

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[cursor]

Cursor Bugbot has reviewed your changes using default effort and found 2 potential issues.

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[cursor]

Stale shutdown aborts FDv2 retry

High Severity

OnFDv2RetryTimer closes the FDv1 adapter while its Next() is still in the orchestrator's WhenAny. That unblocks the old future with Shutdown, which is posted onto the executor after the new FDv2 synchronizer has already been started. OnSynchronizerResult then treats that stale Shutdown as belonging to the replacement and resets active_synchronizer_, so the SDK is left with no live source after the TTL elapses.

Additional Locations (1)

• libs/client-sdk/src/data_sources/fdv2/fdv2_data_source.cpp#L510-L514

 

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[cursor]

Fallback status skips shutdown fence

Medium Severity

When a fallback directive arrives and no FDv1 tier is configured, the new path writes kInterrupted through status_manager_->SetState instead of PublishState. That skips GuardShutdown, so a late write after Close can overwrite the shared status that outlives this source.

Additional Locations (1)

• libs/client-sdk/src/data_sources/fdv2/fdv2_data_source.cpp#L532-L538

 

^{Triggered by learned rule: Client FDv2DataSource: basis is selector; GuardShutdown fences store}

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