feat: migrate tests to use centralized aitbc package utilities

- Migrate HTTP client usage from httpx/requests to aitbc.AITBCHTTPClient
- Update test_payment_integration.py to use AITBCHTTPClient and get_logger
- Fix typo in test_cross_node_blockchain.py (NetworkErroration -> NetworkError)
- Add aitbc validators to test_model_validation.py (validate_address, validate_hash)
- conftest.py already uses aitbc path utilities (get_data_path, get_log_path)
- Other test files already migrated (test_tx_import, test_simple_import, test_minimal, test_block_import_complete, verify_transactions_fixed)

tests/verification/test_cross_node_blockchain.py

@@ -8,7 +8,7 @@ import hashlib
 import subprocess
 from datetime import datetime
 import time
-from aitbc import AITBCHTTPClient, NetworkErroration
+from aitbc import AITBCHTTPClient, NetworkError
 
 # Test configuration
 NODES = {