chore: Add FDv2-compatible contract test support

[keelerm84]

Note

Introduces FDv2 data system configuration and robust synchronizer lifecycle (start/stop), integrates SSE streaming with payload filter, implements an in-memory feature store, and updates tests and contract client accordingly.

• FDv2/Data System:
  • Add Synchronizer.stop() to interface and implement stop/lifecycle management in StreamingDataSource and PollingDataSource.
  • Enhance FDv2 to track/stop the active synchronizer safely with locks; ensure threads shut down cleanly.
  • Add datasystem.config builders (polling_ds_builder, streaming_ds_builder), expose fdv1_fallback_synchronizer in config.
• Streaming:
  • Switch to ld_eventsource.SSEClient; include payload filter in stream URI.
  • Handle stream errors by interrupting/closing SSE; stop on unrecoverable errors; ensure closure on exit.
• Polling:
  • Add stoppable sync loop with _stop flag and stop() method.
• Store:
  • Implement thread-safe InMemoryFeatureStore with basic CRUD, init, and diagnostics; integrate with Store.
• Contract tests:
  • Support FDv2 dataSystem config (initializers/synchronizers, payloadFilter) in client_entity.py.
• Tests:
  • Update streaming synchronizer tests for new SSE client usage and stop/interrupt behavior.

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[kinyoklion]

I assume no threading concerns in this implementation?

[keelerm84]

I don't believe there should be. The data system starts it's own thread which creates and destroys synchronizers synchronously.