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/processor.go

@@ -3,6 +3,7 @@ package imgcache
 import (
 	"bytes"
 	"context"
+	"errors"
 	"fmt"
 	"image"
 	"image/gif"
@@ -14,6 +15,13 @@ import (
 	"golang.org/x/image/webp"
 )
 
+// MaxInputDimension is the maximum allowed width or height for input images.
+// Images larger than this are rejected to prevent DoS via decompression bombs.
+const MaxInputDimension = 8192
+
+// ErrInputTooLarge is returned when input image dimensions exceed MaxInputDimension.
+var ErrInputTooLarge = errors.New("input image dimensions exceed maximum")
+
 // ImageProcessor implements the Processor interface using pure Go libraries.
 type ImageProcessor struct{}