Add failing tests for input dimension and path traversal validation

Tests for:
- ErrInputTooLarge when input image exceeds MaxInputDimension
- ErrPathTraversal for ../, encoded traversal, backslashes, null bytes

internal/imgcache/urlparser.go

@@ -16,6 +16,7 @@ var (
 	ErrInvalidSize       = errors.New("invalid size format")
 	ErrInvalidFormat     = errors.New("invalid or unsupported format")
 	ErrDimensionTooLarge = errors.New("dimension exceeds maximum")
+	ErrPathTraversal     = errors.New("path traversal detected")
 )
 
 // MaxDimension is the maximum allowed width or height.