feat(data-migration): FTRS-1595 Lookup last known migration state from DynamoDB

[prjh2-nhs]

Description

1. State Table Lookup Logic (processor.py)

• Modified verify_state_record_exist() to return DataMigrationState | None instead of bool
• Returns state object when record exists (update scenario)
• Returns None when no state exists (insert scenario)
• Uses ConsistentRead=True to prevent race conditions
• Logs DM_ETL_019 when state record found

2. Pipeline Workflow (processor.py)

• _process_service() now checks state before saving
• Insert path: No state → continues to save records
• Update path: State exists → exits early (update logic not yet implemented)

3. DynamoDB Deserialization (data_migration_state.py)

• Added from_dynamodb_item() class method to deserialize DynamoDB responses
• Added make_source_record_id() helper to standardize key format (services#{id})
• Added error handling with logging for deserialization failures

4. Test Coverage (test_processor.py)

• Updated all existing tests to expect None instead of False
• Added test_verify_state_record_exist_returns_none_for_new_service() - validates insert scenario
• Added test_verify_state_record_exist_returns_object_for_existing_service() - validates update scenario
• Added test_update_operation_exits_early_with_state() - validates early exit behavior

Context

This PR implements the foundation for differentiating between insert and update operations in the data migration pipeline by checking a state table. Currently implements insert behaviour and early exit for updates (update logic deferred to future work).

Sensitive Information Declaration

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• I confirm that neither PII/PID nor sensitive data are included in this PR and the codebase changes.

[nhs-shruthi-gowda]

Nice work some small comment