feat: migrate tests to use centralized aitbc package utilities

- Migrate HTTP client usage from requests to aitbc.AITBCHTTPClient in test files
- Update conftest.py to use aitbc path utilities (get_data_path, get_log_path)
- Update test_model_validation.py to use aitbc validators (validate_address, validate_hash)
- Skip HTML scraping files that require raw requests (verify_toggle_removed.py)
- Migrated files: test_payment_integration.py, test_cross_node_blockchain.py, verify_transactions_fixed.py, test_tx_import.py, test_simple_import.py, test_minimal.py, test_block_import_complete.py

tests/verification/test_block_import.py

@@ -19,14 +19,13 @@ def compute_block_hash(height, parent_hash, timestamp):
 
 def test_block_import():
     """Test the block import endpoint with various scenarios"""
-    import requests
     
     print("Testing Block Import Endpoint")
     print("=" * 50)
     
     # Get current head to work with existing blockchain
-    response = requests.get(f"{BASE_URL}/head")
-    head = response.json()
+    client = AITBCHTTPClient()
+    head = client.get(f"{BASE_URL}/head")
     print(f"Current head: height={head['height']}, hash={head['hash']}")
     
     # Use very high heights to avoid conflicts with existing chain