fix: mypy checks violation in AzureBlobStorage (#13215)

api/extensions/storage/azure_blob_storage.py

 from collections.abc import Generator
 from datetime import UTC, datetime, timedelta
+from typing import Optional
 
-from azure.identity import DefaultAzureCredential
+from azure.identity import ChainedTokenCredential, DefaultAzureCredential
 from azure.storage.blob import AccountSasPermissions, BlobServiceClient, ResourceTypes, generate_account_sas
 
 from configs import dify_config
         self.account_name = dify_config.AZURE_BLOB_ACCOUNT_NAME
         self.account_key = dify_config.AZURE_BLOB_ACCOUNT_KEY
 
+        self.credential: Optional[ChainedTokenCredential] = None
         if self.account_key == "managedidentity":
             self.credential = DefaultAzureCredential()
         else:
 
     def _sync_client(self):
         if self.account_key == "managedidentity":
-            return BlobServiceClient(account_url=self.account_url, credential=self.credential)
+            return BlobServiceClient(account_url=self.account_url, credential=self.credential)  # type: ignore
 
         cache_key = "azure_blob_sas_token_{}_{}".format(self.account_name, self.account_key)
         cache_result = redis_client.get(cache_key)