fix: remove ibm-db dependency and refactor import order (#10330)

### What problem does this PR solve?
issue： 
#10326
change：
 remove ibm-db dependency and refactor import order

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

agent/tools/exesql.py

@@ -21,7 +21,6 @@ import pandas as pd
 import pymysql
 import psycopg2
 import pyodbc
-import ibm_db
 from agent.tools.base import ToolParamBase, ToolBase, ToolMeta
 from api.utils.api_utils import timeout
 
@@ -125,6 +124,7 @@ class ExeSQL(ToolBase, ABC):
             )
             db = pyodbc.connect(conn_str)
         elif self._param.db_type == 'IBM DB2':
+            import ibm_db
             conn_str = (
                 f"DATABASE={self._param.database};"
                 f"HOSTNAME={self._param.host};"