feat(consensus): implement transaction processing in PoA block proposer

- Process transactions from mempool during block proposal
- Update sender and recipient account balances
- Create transaction records with confirmed status
- Include transaction hashes in block hash computation
- Update tx_count in blocks based on processed transactions
- Add balance validation and nonce management
- Handle transaction failures gracefully with logging
- Fix get_balance endpoint to use chain_id helper
- Update

apps/blockchain-node/src/aitbc_chain/rpc/router.py

@@ -312,6 +312,7 @@ async def get_receipts(limit: int = 20, offset: int = 0) -> Dict[str, Any]:
 
 @router.get("/getBalance/{address}", summary="Get account balance")
 async def get_balance(address: str, chain_id: str = None) -> Dict[str, Any]:
+    chain_id = get_chain_id(chain_id)
     metrics_registry.increment("rpc_get_balance_total")
     start = time.perf_counter()
     with session_scope() as session: