Add failing test for WebP encoding support

TDD: This test expects WebP encoding to succeed. It currently fails
because WebP encoding is not implemented (returns ErrUnsupportedOutputFormat).
The test will pass once we add the gen2brain/webp library.

internal/imgcache/processor_test.go

@@ -313,7 +313,7 @@ func TestImageProcessor_AcceptsMaxDimensionInput(t *testing.T) {
 	defer result.Content.Close()
 }
 
-func TestImageProcessor_RejectsUnsupportedOutputFormat_WebP(t *testing.T) {
+func TestImageProcessor_EncodeWebP(t *testing.T) {
 	proc := NewImageProcessor()
 	ctx := context.Background()
 
@@ -322,17 +322,37 @@ func TestImageProcessor_RejectsUnsupportedOutputFormat_WebP(t *testing.T) {
 	req := &ImageRequest{
 		Size:    Size{Width: 100, Height: 75},
 		Format:  FormatWebP,
-		Quality: 85,
+		Quality: 80,
 		FitMode: FitCover,
 	}
 
-	_, err := proc.Process(ctx, bytes.NewReader(input), req)
-	if err == nil {
-		t.Fatal("Process() should return error for unsupported WebP encoding")
+	result, err := proc.Process(ctx, bytes.NewReader(input), req)
+	if err != nil {
+		t.Fatalf("Process() error = %v, want nil", err)
+	}
+	defer result.Content.Close()
+
+	// Verify output is valid WebP
+	data, err := io.ReadAll(result.Content)
+	if err != nil {
+		t.Fatalf("failed to read result: %v", err)
 	}
 
-	if !errors.Is(err, ErrUnsupportedOutputFormat) {
-		t.Errorf("Process() error = %v, want ErrUnsupportedOutputFormat", err)
+	mime, err := DetectFormat(data)
+	if err != nil {
+		t.Fatalf("DetectFormat() error = %v", err)
+	}
+
+	if mime != MIMETypeWebP {
+		t.Errorf("Output format = %v, want %v", mime, MIMETypeWebP)
+	}
+
+	// Verify dimensions
+	if result.Width != 100 {
+		t.Errorf("Width = %d, want 100", result.Width)
+	}
+	if result.Height != 75 {
+		t.Errorf("Height = %d, want 75", result.Height)
 	}
 }