Fix chain import/export to handle duplicates, add metadata fields, and improve datetime parsing

- Add chain_id field to Block creation in import_block endpoint
- Remove await from synchronous session.commit in import_block
- Add _serialize_optional_timestamp helper to handle various timestamp formats
- Add _parse_datetime_value helper with proper datetime parsing and error handling
- Add _select_export_blocks to filter duplicate blocks by height during export
- Add _dedupe_import_blocks to filter

apps/blockchain-node/src/aitbc_chain/sync.py

@@ -16,6 +16,7 @@ from sqlmodel import Session, select
 from .config import settings
 from .logger import get_logger
 from .state.merkle_patricia_trie import StateManager
+from .state.state_transition import get_state_transition
 from .metrics import metrics_registry
 from .models import Block, Account
 from aitbc_chain.models import Transaction as ChainTransaction