Migrate Microsoft.IdentityModel.TestExtensions from Newtonsoft.Json to System.Text.Json

[Copilot]

• Analyze the current usage of Newtonsoft.Json in TestTokenCreator.cs
• Replace Newtonsoft.Json imports with System.Text.Json in TestTokenCreator.cs
• Replace JObject and JToken usage with System.Text.Json equivalents in CreateJsonPayload method
• Update Microsoft.IdentityModel.TestExtensions.csproj to remove Newtonsoft.Json reference
• Add System.Text.Json reference to Microsoft.IdentityModel.TestExtensions.csproj with proper conditional targeting
• Add IL2026 and IL3050 to NoWarn to suppress AOT warnings (TestExtensions is a test utility library)
• Add explanatory comment for NoWarn suppressions in csproj file
• Build the TestExtensions project to verify compilation
• Run tests that use TestTokenCreator to ensure functionality is preserved
• Verify no other files in TestExtensions use Newtonsoft.Json

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Feature Request] Migrate Microsoft.IdentityModel.TestExtensions to System.Text.Json</issue_title>
<issue_description>Is your feature request related to a problem? Please describe.

Microsoft.IdentityModel.TestExtensions should be migrated off of Newtonsoft.Json and instead should use System.Text.Json.

• TestTokenCreator.cs should replace usage of Newtonsoft.Json with System.Text.Json
• Microsoft.IdentityModel.TestExtensions.csproj should remove the reference for Newtonsoft.Json and add a reference to System.Text.Json as so:

<ItemGroup Condition="'$(TargetFrameworkIdentifier)' != '.NETCoreApp'">
--
<PackageReference Include="System.Text.Json" Version="$(SystemTextJson)" />
</ItemGroup>

</issue_description>

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

Fixes #3355

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[kllysng]

The changes for this package lgtm. Question - will we do this for the other packages too?

[keegan-caruso]

The changes for this package lgtm. Question - will we do this for the other packages too?

I believe this is the only src/ package using Newtonsoft through a nuget package. There are many examples in test/ that would be good to clean up later.