Snapshot the generated openapi as part of the OpenApi source generator tests

Author: desjoerd

src/OpenApi/test/Microsoft.AspNetCore.OpenApi.SourceGenerators.Tests/SnapshotTestHelper.cs

@@ -200,6 +200,14 @@ void OnEntryPointExit(Exception exception)
             using var writer = new FormattingStreamWriter(stream, CultureInfo.InvariantCulture) { AutoFlush = true };
             var targetMethod = serviceType.GetMethod("GenerateAsync", [typeof(string), typeof(TextWriter)]) ?? throw new InvalidOperationException("Could not resolve GenerateAsync method.");
             targetMethod.Invoke(service, ["v1", writer]);
+
+            var openApiString = Encoding.UTF8.GetString(stream.ToArray());
+            await Verifier.Verify(openApiString)
+                .UseTextForParameters("openapi.json")
+                .UseDirectory(SkipOnHelixAttribute.OnHelix()
+                    ? Path.Combine(Environment.GetEnvironmentVariable("HELIX_WORKITEM_ROOT"), "snapshots")
+                    : "snapshots");
+
             stream.Position = 0;
             var (document, _) = await OpenApiDocument.LoadAsync(stream, "json");
             verifyFunc(document);