Use JSON source generator in WebApplicationFactory

[martincostello]

Use JSON source generator in WebApplicationFactory

Use JSON source generator to deserialize dictionary.

Description

Use JSON source generator to deserialize JSON to prevent InvalidOperationException in a test project with JsonSerializerIsReflectionEnabledByDefault=false.

I wasn't sure how to test this without having to add an entirely new test project - happy to add something if someone can give a suggestion on how to go about that in a way that would be acceptable to the team.

Fixes #55586

[amcasey]

Just noticed that the old labels were set manually. Why networking, @mkArtakMSFT?

[amcasey]

FYI @captainsafia

[captainsafia]

@martincostello TY for the PR.

We reference the Mvc.Testing package in other tests in the repo (Identity.FunctionalTests, MvcFunctionalTest, for example). We can probably verify whether or not this fix applies by setting the JsonSerializerIsReflectionEnabledByDefault property in those projects and checking if the change resolves any IOE.

[martincostello]

Playing around with Microsoft.AspNetCore.Mvc.FunctionalTests locally I get the following results:

JSON source generator?	JsonSerializerIsReflectionEnabledByDefault	Passed	Failed
No	true	1579	2
No	false	4	1577
Yes	true	1579	2
Yes	false	1469	112

The tests that fail for me regardless of the settings are from AntiforgeryMiddlewareTest for Works_WithAntiforgeryMetadata_ValidToken and Works_WithAntiforgeryMetadata_ValidToken_DisableRequestSizeLimits - I'm not sure why.

[martincostello]

The tests that fail for me regardless of the settings are from AntiforgeryMiddlewareTest for Works_WithAntiforgeryMetadata_ValidToken and Works_WithAntiforgeryMetadata_ValidToken_DisableRequestSizeLimits - I'm not sure why.

#55779