fix: parent block validation in importBlock + no retry on 4xx in AITBCHTTPClient

- importBlock now returns 400 'Parent block not found' when parent_hash
  is not in the DB (skipped for genesis height 1)
- AITBCHTTPClient._retry_request now immediately re-raises HTTPError for
  4xx responses instead of retrying - fixes RetryError wrapping 400/409
  in test_block_import_complete

aitbc/network/http_client.py

@@ -163,6 +163,19 @@ class AITBCHTTPClient:
                     time.sleep(backoff_time)
                 
                 return request_func(*args, **kwargs)
+            except requests.HTTPError as e:
+                # Never retry client errors (4xx) - they are deterministic
+                if e.response is not None and 400 <= e.response.status_code < 500:
+                    raise
+                last_error = e
+                if attempt < self.max_retries:
+                    if self.enable_logging:
+                        self.logger.warning(f"Request failed (attempt {attempt + 1}/{self.max_retries + 1}): {e}")
+                    continue
+                else:
+                    if self.enable_logging:
+                        self.logger.error(f"All retry attempts exhausted: {e}")
+                    raise RetryError(f"Retry attempts exhausted: {e}")
             except requests.RequestException as e:
                 last_error = e
                 if attempt < self.max_retries:

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

@@ -912,6 +912,18 @@ async def import_block(
                         detail=f"Block height {block_height} already exists with different hash",
                     )
 
+                # Validate parent block exists (skip for genesis block height 1)
+                parent_hash = block_data["parent_hash"]
+                if block_height > 1:
+                    parent_block = session.exec(
+                        select(Block).where(Block.hash == parent_hash)
+                    ).first()
+                    if parent_block is None:
+                        raise HTTPException(
+                            status_code=status.HTTP_400_BAD_REQUEST,
+                            detail="Parent block not found",
+                        )
+
                 # Check for hash conflicts across chains
                 existing_block = session.execute(
                     select(Block).where(Block.hash == block_hash)