Add shared-root destination state model

internal/config/validate_test.go

@@ -141,6 +141,40 @@ func TestValidateReconciliationPolicy(t *testing.T) {
 	}
 }
 
+func TestValidateStatePolicy(t *testing.T) {
+	tests := []struct {
+		name    string
+		mode    string
+		wantErr bool
+	}{
+		{name: "single owner", mode: StateModeSingleOwner},
+		{name: "shared root", mode: StateModeSharedRoot},
+		{name: "invalid", mode: "shared", wantErr: true},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			cfg := Config{Pipelines: []Pipeline{{
+				ID:     "reports",
+				Source: Backend{Backend: BackendLocal, Path: "/source"},
+				Destinations: []Destination{{
+					ID:      "archive",
+					Backend: BackendLocal,
+					Path:    "/destination",
+					State:   StatePolicy{Mode: tt.mode},
+				}},
+			}}}
+			ApplyDefaults(&cfg)
+			err := Validate(cfg)
+			if tt.wantErr && err == nil {
+				t.Fatal("Validate() error = nil, want error")
+			}
+			if !tt.wantErr && err != nil {
+				t.Fatalf("Validate() error = %v", err)
+			}
+		})
+	}
+}
+
 func TestValidateLinks(t *testing.T) {
 	tests := []struct {
 		name    string