Add type stubs for cachetools to fix mypy errors

Added types-cachetools to dev dependencies to resolve mypy type checking errors:
  - Library stubs not installed for 'cachetools'
  - Returning Any from function declared to return 'ValidationResult | None'

This ensures clean mypy type checking in CI builds.

Author: beonde

pyproject.toml

@@ -43,6 +43,7 @@ dev = [
     "black>=24.0.0",
     "ruff>=0.3.0",
     "mypy>=1.9.0",
+    "types-cachetools>=5.3.0",
 ]
 
 [project.urls]