Harden sample authentication and storage access

[abaranwal-ms]

Summary

• remove the .NET sample's global TLS certificate validation bypass
• keep the Python sample VHD container private and use a short-lived read-only SAS for image creation
• update the legacy .NET sample Newtonsoft.Json references so the sample restores and builds cleanly

Validation

• dotnet build dotnetSamples/LocalArm/CreateVmSample/CreateVmSample.sln
• python -m py_compile Scripts/PythonScript/example_ase.py

Notes

• the .NET sample executable still requires real Azure Stack Edge configuration values for end-to-end runtime testing
• the Python sample still requires a live Azure Stack Edge environment and credentials for end-to-end execution