[status_flag] Fix bad stringifier (#37014)

Also add a test for ValueOrFailure stringification

Closes #37014

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/37014 from ctiller:status_flag_fix 8194ca3ea8
PiperOrigin-RevId: 645438082

src/core/lib/promise/status_flag.h

@@ -263,7 +263,7 @@ class ValueOrFailure {
   friend void AbslStringify(Sink& sink, const ValueOrFailure& value) {
     if (value.ok()) {
       sink.Append("Success(");
-      sink.Append(absl::StrCat(value));
+      sink.Append(absl::StrCat(*value));
       sink.Append(")");
     } else {
       sink.Append("Failure");

test/core/promise/status_flag_test.cc

@@ -38,6 +38,11 @@ TEST(StatusFlagTest, Basics) {
   EXPECT_FALSE(IsStatusOk(Failure{}));
 }
 
+TEST(StatusFlagTest, StringifyValueOrFailure) {
+  EXPECT_EQ(absl::StrCat(ValueOrFailure<int>(42)), "Success(42)");
+  EXPECT_EQ(absl::StrCat(ValueOrFailure<int>(Failure{})), "Failure");
+}
+
 }  // namespace grpc_core
 
 int main(int argc, char** argv) {