docs: Update VCR testing documentation and fix PEP 8 import order

- Update docs/vcr-testing.md with new PII sanitization features
- Document transport_helpers.inject_transport() for simpler test setup
- Add sanitize_cassettes.py script documentation
- Update file structure to include all new components
- Fix PEP 8: Move copy import to top of openai_compatible.py
- Enhance security notes about automatic sanitization

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

providers/openai_compatible.py

@@ -1,6 +1,7 @@
 """Base class for OpenAI-compatible API providers."""
 
 import base64
+import copy
 import ipaddress
 import logging
 import os
@@ -283,8 +284,6 @@ class OpenAICompatibleProvider(ModelProvider):
         Returns:
             dict: Sanitized copy of parameters safe for logging
         """
-        import copy
-
         sanitized = copy.deepcopy(params)
 
         # Sanitize messages content