feat: split client tooling and add OAuth server login

go/internal/server/stateio/json_test.go

@@ -0,0 +1,18 @@
+package stateio
+
+import "testing"
+
+func TestRequireExactJSONObject(t *testing.T) {
+	if err := RequireExactJSONObject([]byte(`{"version":1,"items":[]}`), "version", "items"); err != nil {
+		t.Fatal(err)
+	}
+	for _, payload := range []string{
+		`{"version":1}`,
+		`{"version":1,"items":[],"legacy":true}`,
+		`[]`,
+	} {
+		if err := RequireExactJSONObject([]byte(payload), "version", "items"); err == nil {
+			t.Fatalf("accepted non-current layout %s", payload)
+		}
+	}
+}