Revert back to JsonElement-backed JsonValue for deserialization

[PranavSenthilnathan]

JsonValueOfElement is more permissive than other classes derived from JsonValue. For example, it allows GetValue<Guid> even when the underlying value's JSON type is a JSON string. The following method has the implementation:

runtime/src/libraries/System.Text.Json/src/System/Text/Json/Nodes/JsonValueOfElement.cs

Line 50 in 2dbdff1

public override bool TryGetValue<TypeToConvert>([NotNullWhen(true)] out TypeToConvert value)

#115856 changed deserialization of primitives to use JsonValue.Create instead of the JsonElement-backed creation, so the people relying on the previous behavior of GetValue will now be broken. This was a small part of that PR and only that part has been reverted in this PR.

In the long term, we can reintroduce this change in a different way: have another class derived from JsonValue that (1) stores the value instead of JsonElement and (2) also allows the same conversions that JsonValueOfElement does. But for now, we revert to get back to baseline.

Fixes #116730

/cc @eiriktsarpalis @jeffhandley I'm planning to cherry pick this to the release branch as well after merging in main.

[Copilot]

Pull Request Overview

This PR reverts a recent change to primitive deserialization in JsonValue that broke the permissiveness of JsonValueOfElement when using GetValue. It restores behavior by reverting to the JsonElement-backed creation and updates the associated test harness to accommodate the change.

• Revert deserialization of primitives to use JsonElement-backed JsonValue
• Update tests to wrap primitive values using a new WrappedT helper type
• Adjust JsonValueConverter to revert to the previous parsing logic

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/libraries/System.Text.Json/tests/System.Text.Json.Tests/JsonNode/JsonValueTests.cs	Updated test signatures to use WrappedT and modified the Wrap helper to wrap values in WrappedT
src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Node/JsonValueConverter.cs	Reverted the primitive value deserialization to use JsonElement-backed JsonValue

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[PranavSenthilnathan]

XUnit was coercing T? to T for some types (like char? to char) so the tests weren't actually running for those cases. It doesn't do that when using the custom type.

[eiriktsarpalis]

In the long term, we can reintroduce this change in a different way: have another class derived from JsonValue that (1) stores the value instead of JsonElement and (2) also allows the same conversions that JsonValueOfElement does. But for now, we revert to get back to baseline.

Any reason why we cannot do this right away? We're not backporting this fix so I think we can afford to do this in a way that both addresses the regression and makes TryGetValue behave consistently regardless of underlying implementation.

[jeffhandley]

If we can get the "long term" fix into Preview 7, I'm supportive of taking that fix instead of this one, and still having the regression in Preview 6.