Fix linting issues: add missing base64 import and remove unused import

- Add missing base64 import in providers/base.py
- Remove unused base64 import from providers/openai_compatible.py
- All tests now pass (19/19 image validation tests)
- Code quality checks pass 100%

providers/base.py

@@ -1,5 +1,6 @@
 """Base model provider interface and data classes."""
 
+import base64
 import binascii
 import logging
 import os

providers/openai_compatible.py

@@ -1,6 +1,5 @@
 """Base class for OpenAI-compatible API providers."""
 
-import base64
 import copy
 import ipaddress
 import logging