Support generation for System.Net.Http.Json

[HavenDV]

https://www.nuget.org/packages/System.Net.Http.Json

[HavenDV]

Verified on main. AutoSDK already supports System.Net.Http.Json generation via --use-system-net-http-json, and commit 86b178b adds CLI integration coverage for the package-backed netstandard2.0 scenario from this issue. Publish workflow for that push succeeded: https://github.com/tryAGI/AutoSDK/actions/runs/24374661801