Fix GetValue<object>() on JsonValueOfJsonPrimitive types

[Copilot]

Description

JsonValueOfJsonString, JsonValueOfJsonBool, and JsonValueOfJsonNumber (introduced in .NET 10) throw InvalidOperationException on GetValue<object>() because their TryGetValue<T> methods don't handle typeof(T) == typeof(object). The older JsonValueOfElement handled this implicitly via if (Value is TypeToConvert element).

var reader = new Utf8JsonReader("\"Hello World!\""u8);
reader.Read();
var node = converter.Read(ref reader, typeof(JsonNode), options);

// .NET 10: throws InvalidOperationException
// .NET 9:  works fine
var o = node.GetValue<object>();

Changes

• JsonValueOfJsonString: Added typeof(T) == typeof(object) to the string branch — returns the decoded string
• JsonValueOfJsonBool: Added typeof(T) == typeof(object) to the bool branch — returns the boxed bool
• JsonValueOfJsonNumber: Added typeof(T) == typeof(object) to the JsonElement branch — returns a JsonElement (no single numeric type to pick; consistent with prior JsonValueOfElement behavior)
• Added TryGetValue<object> assertions to existing tests and a new GetValue_Object theory exercising the converter code path

Original prompt

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This section details on the original issue you should resolve

<issue_title>Breaking change in JsonNodeConverter</issue_title>
<issue_description>### Description

Using a JsonConverter to read a string node, .NET 9 and .NET 10 show different behavior.

The new System.Text.Json.Nodes.JsonValueOfJsonString introduced in .NET 10 cannot be converted to an object using GetValue<object>(), resulting in an exception.

Reproduction Steps

See Repo at https://github.com/NiceWaffel/jsonnodeconverter-repro

var reader = new Utf8JsonReader("\"Hello World!\""u8);
reader.Read();
var node = (SerializationContext.Default.JsonNode.Converter as JsonConverter<JsonNode>)
  ?.Read(ref reader, typeof(JsonNode), SerializationContext.Default.Options);

Debug.Assert(node != null, "We should get a valid JsonNode.");

// Different type in .NET 8, .NET 9 and .NET 10
// .NET 8: System.Text.Json.Nodes.JsonValuePrimitive`1[System.Text.Json.JsonElement]
// .NET 9: System.Text.Json.Nodes.JsonValueOfElement
// .NET 10: System.Text.Json.Nodes.JsonValueOfJsonString
Console.WriteLine(node.GetType());

// Exception in .NET 10 (no exception in .NET 8)
// System.InvalidOperationException: An element of type 'String' cannot be converted to a 'System.Object'.
var o = node.GetValue<object>();
Console.WriteLine(o);

Expected behavior

The behavior in .NET 8, .NET 9 and .Net 10 should be the same. No exception should be thrown.

Actual behavior

In .NET 10, a string JsonNode (read as JsonValueOfJsonString) cannot be converted to an object, resulting in an exception.

Unhandled exception. System.InvalidOperationException: An element of type 'String' cannot be converted to a 'System.Object'.
   at System.Text.Json.ThrowHelper.ThrowInvalidOperationException_NodeUnableToConvertElement(JsonValueKind valueKind, Type destinationType)
   at System.Text.Json.Nodes.JsonValueOfJsonString.GetValue[T]()

Regression?

No response

Known Workarounds

No response

Configuration

No response

Other information

JsonValueOfJsonString was introduced with #116798</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Breaking change in JsonNodeConverter #123372

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