Add OIDC token exchange for NuGet trusted publishing

[Aaronontheweb]

Summary

The publish_nuget.yml workflow had id-token: write permission but was never exchanging the GitHub OIDC token for a temporary NuGet API key. As a result, dotnet nuget push fell back to the v2 endpoint and failed with a 401.

Changes

• Add NuGet/login@v1 step to exchange OIDC token for short-lived API key
• Pass the temp key via --api-key to both dotnet nuget push commands

Root Cause

Without the NuGet/login step, there was no API key in scope. dotnet nuget push with -s but no --api-key doesn't use the v3 endpoint for trusted publishing — it falls back to v2 (https://www.nuget.org/api/v2/package) which requires a traditional API key.

Fixes the Publish NuGet CI job failure from run #24851085466.