Add failing test for AVIF decoding

Test verifies that AVIF images can be decoded and processed.
Currently fails because AVIF decoding is not implemented.

internal/imgcache/processor_test.go

@@ -9,6 +9,7 @@ import (
 	"image/jpeg"
 	"image/png"
 	"io"
+	"os"
 	"testing"
 )
 
@@ -416,6 +417,46 @@ func TestImageProcessor_EncodeWebP(t *testing.T) {
 	}
 }
 
+func TestImageProcessor_DecodeAVIF(t *testing.T) {
+	proc := NewImageProcessor()
+	ctx := context.Background()
+
+	// Load test AVIF file
+	input, err := os.ReadFile("testdata/red.avif")
+	if err != nil {
+		t.Fatalf("failed to read test AVIF: %v", err)
+	}
+
+	// Request resize and convert to JPEG
+	req := &ImageRequest{
+		Size:    Size{Width: 2, Height: 2},
+		Format:  FormatJPEG,
+		Quality: 85,
+		FitMode: FitCover,
+	}
+
+	result, err := proc.Process(ctx, bytes.NewReader(input), req)
+	if err != nil {
+		t.Fatalf("Process() error = %v, want nil (AVIF decoding should work)", err)
+	}
+	defer result.Content.Close()
+
+	// Verify output is valid JPEG
+	data, err := io.ReadAll(result.Content)
+	if err != nil {
+		t.Fatalf("failed to read result: %v", err)
+	}
+
+	mime, err := DetectFormat(data)
+	if err != nil {
+		t.Fatalf("DetectFormat() error = %v", err)
+	}
+
+	if mime != MIMETypeJPEG {
+		t.Errorf("Output format = %v, want %v", mime, MIMETypeJPEG)
+	}
+}
+
 func TestImageProcessor_RejectsUnsupportedOutputFormat_AVIF(t *testing.T) {
 	proc := NewImageProcessor()
 	ctx := context.Background()