fix: add hash format validation to importBlock endpoint

- Validate block hash is 0x + 64 hex chars; return 400 for invalid format
- Fixes test_block_import and test_block_import_complete CI failures
- Update test_block_import_complete Test 2 to expect 400 (invalid hash format)
  instead of 409 (conflict); invalid hash is now rejected before DB lookup
- Move re import to top of router.py

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

@@ -3,6 +3,7 @@ from __future__ import annotations
 import asyncio
 import hashlib
 import json
+import re
 import time
 import uuid
 from typing import Any, Dict, Optional, List
@@ -874,6 +875,10 @@ async def import_block(
             chain_id = block_data.get("chain_id") or block_data.get("chainId") or get_chain_id(None)
             block_hash = block_data["hash"]
 
+            # Validate block hash format: must be 0x followed by exactly 64 hex characters
+            if not isinstance(block_hash, str) or not re.fullmatch(r"0x[0-9a-fA-F]{64}", block_hash):
+                raise HTTPException(status_code=status.HTTP_400_BAD_REQUEST, detail="Invalid block hash format")
+
             try:
                 block_height = int(block_data["height"])
             except (KeyError, TypeError, ValueError) as exc: