chore: Reorganize import structure to align with compatibility expectations

[keelerm84]

• temp debugging
• chore: Reorganize import structure to align with compatibility expectations

---

Note

Consolidates FDv2 types into ldclient.interfaces, introduces datasystem builder APIs (with optional FDv1 fallback), updates polling/streaming and file/test data sources, and refreshes FDv2 system/persistence and tests.

• Core API/Interfaces:
  • Move FDv2 protocol and lifecycle types (Selector, ChangeSet, Basis, Update, Initializer, Synchronizer, EventName, ObjectKind, etc.) into ldclient.interfaces; update imports across modules.
  • Trim ldclient.impl.datasystem.protocolv2 to data payload types (PutObject, DeleteObject, Goodbye, Error).
• Data System Builders (ldclient/datasystem.py):
  • Add ConfigBuilder with initializers, synchronizers, fdv1_compatible_synchronizer, and data_store.
  • Provide default, streaming, polling, and custom presets; expose polling_ds_builder, streaming_ds_builder, and fdv1_fallback_ds_builder.
• Data Sources:
  • Update polling/streaming to implement Initializer/Synchronizer; support env ID propagation and FDv1 fallback via headers (_LD_ENVID_HEADER, _LD_FD_FALLBACK_HEADER).
  • Remove impl/datasourcev2/status.py; inline status handling where used.
• FDv2 System:
  • Refactor FDv2 to include inlined data source/store status providers, persistent store wrapper, outage recovery, and synchronizer fallback/recovery logic (including FDv1 fallback path).
  • Make FDv1/FDv2 implement DataSystem.
• Integrations:
  • Add Files.new_data_source_v2 builder and implement file-based FDv2 initializer/synchronizer.
  • Add TestDataV2-based FDv2 initializer/synchronizer implementations.
• Contract tests:
  • Update contract-tests/client_entity.py to use ldclient.datasystem builders and configure optional FDv1 fallback.
• Config:
  • Import Initializer/Synchronizer from ldclient.interfaces; keep DataSystemConfig as the shared config carrier.
• Tests:
  • Update and expand tests for polling/streaming parsers, FDv2 datasystem behavior (fallbacks, persistence, env ID), file/test data sources; remove obsolete status tests.

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