Mutating results of JsonSerializerContext.GetTypeInfo can result in corruption of static metadata properties

[layomia]

    A more insidious instantiation of the bug above is that mutating results of `JsonSerializerContext.GetTypeInfo` can result in corruption of static metadata properties, as illustrated by the example below:

JsonTypeInfo foo = MySerializerContext.Default.GetTypeInfo(typeof(MyClass));
foo.Properties.Clear();

Console.WriteLine(MySerializerContext.Default.MyClass.Properties.Count); // 0, properties dropped from a seemingly unrelated instance

[JsonSerializable(typeof(MyClass))]
public partial class MySerializerContext : JsonSerializerContext
{
}
public class MyClass
{
    public int A0 { get; set; }
}

Bare minimum, we should make sure that modifications don't allow cross-contamination in .NET 7.

Originally posted by @eiriktsarpalis in #76535 (comment)

[ghost]

Tagging subscribers to this area: @dotnet/area-system-text-json, @gregsdennis
See info in area-owners.md if you want to be subscribed.

Issue Details

---

    A more insidious instantiation of the bug above is that mutating results of `JsonSerializerContext.GetTypeInfo` can result in corruption of static metadata properties, as illustrated by the example below:

JsonTypeInfo foo = MySerializerContext.Default.GetTypeInfo(typeof(MyClass));
foo.Properties.Clear();

Console.WriteLine(MySerializerContext.Default.MyClass.Properties.Count); // 0, properties dropped from a seemingly unrelated instance

[JsonSerializable(typeof(MyClass))]
public partial class MySerializerContext : JsonSerializerContext
{
}
public class MyClass
{
    public int A0 { get; set; }
}

Bare minimum, we should make sure that modifications don't allow cross-contamination in .NET 7.

Originally posted by @eiriktsarpalis in #76535 (comment)

Author:	layomia
Assignees:	layomia
Labels:	area-System.Text.Json, untriaged
Milestone:	-

[layomia]

Same fix as #76535.