refactor: remove __all__ from internal cache module

- Remove __all__ export as this is an internal module
- Add module docstring describing package purpose

src/crewai/agents/cache/__init__.py

@@ -1,3 +1,5 @@
-from .cache_handler import CacheHandler
+"""Internal caching utilities for agent tool execution.
 
-__all__ = ["CacheHandler"]
+This package provides caching mechanisms for storing and retrieving
+tool execution results to avoid redundant operations.
+"""