Pruefe Layout-Validierung systematischer

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

server/backend/internal/campaigns/messagewall/resolver_test.go

@@ -1,6 +1,9 @@
 package messagewall
 
-import "testing"
+import (
+	"strings"
+	"testing"
+)
 
 func TestResolveBuildsScenes(t *testing.T) {
 	request := ResolveRequest{
@@ -60,3 +63,87 @@ func TestValidateRejectsOverlappingSlots(t *testing.T) {
 		t.Fatal("Validate() error = nil, want overlap error")
 	}
 }
+
+func TestValidateRejectsDocumentedInvalidLayouts(t *testing.T) {
+	tests := []struct {
+		name        string
+		layout      Layout
+		wantMessage string
+	}{
+		{
+			name: "unsupported version",
+			layout: Layout{
+				Version:         2,
+				CoordinateSpace: CoordinateSpace{Width: 100, Height: 100, Unit: "grid"},
+				FitMode:         "cover",
+				Slots:           []Slot{{SlotID: "a", X: 0, Y: 0, Width: 100, Height: 100}},
+			},
+			wantMessage: "unsupported layout version",
+		},
+		{
+			name: "unsupported unit",
+			layout: Layout{
+				Version:         1,
+				CoordinateSpace: CoordinateSpace{Width: 100, Height: 100, Unit: "pixel"},
+				FitMode:         "cover",
+				Slots:           []Slot{{SlotID: "a", X: 0, Y: 0, Width: 100, Height: 100}},
+			},
+			wantMessage: "unsupported coordinate_space unit",
+		},
+		{
+			name: "unsupported fit mode",
+			layout: Layout{
+				Version:         1,
+				CoordinateSpace: CoordinateSpace{Width: 100, Height: 100, Unit: "grid"},
+				FitMode:         "stretch",
+				Slots:           []Slot{{SlotID: "a", X: 0, Y: 0, Width: 100, Height: 100}},
+			},
+			wantMessage: "unsupported fit_mode",
+		},
+		{
+			name: "empty slots",
+			layout: Layout{
+				Version:         1,
+				CoordinateSpace: CoordinateSpace{Width: 100, Height: 100, Unit: "grid"},
+				FitMode:         "cover",
+			},
+			wantMessage: "layout must contain at least one slot",
+		},
+		{
+			name: "duplicate slot ids",
+			layout: Layout{
+				Version:         1,
+				CoordinateSpace: CoordinateSpace{Width: 100, Height: 100, Unit: "grid"},
+				FitMode:         "cover",
+				Slots: []Slot{
+					{SlotID: "a", X: 0, Y: 0, Width: 50, Height: 100},
+					{SlotID: "a", X: 50, Y: 0, Width: 50, Height: 100},
+				},
+			},
+			wantMessage: "duplicate slot_id",
+		},
+		{
+			name: "slot exceeds bounds",
+			layout: Layout{
+				Version:         1,
+				CoordinateSpace: CoordinateSpace{Width: 100, Height: 100, Unit: "grid"},
+				FitMode:         "cover",
+				Slots:           []Slot{{SlotID: "a", X: 60, Y: 0, Width: 50, Height: 100}},
+			},
+			wantMessage: "exceeds coordinate_space width",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := Validate(tt.layout)
+			if err == nil {
+				t.Fatal("Validate() error = nil, want validation error")
+			}
+
+			if !strings.Contains(err.Error(), tt.wantMessage) {
+				t.Fatalf("Validate() error = %q, want message containing %q", err.Error(), tt.wantMessage)
+			}
+		})
+	}
+}