Add NuGet package packing and artifact upload to CI workflow

[paulirwin]

Summary

• Automatically pack NuGet packages on successful builds
• Upload packages as artifacts for easy download from build details page
• Only runs on non-PR builds and Windows builds to avoid duplication

Test plan

• Verify NuGet packages are packed and uploaded on successful main/develop branch builds
• Verify the step is skipped on pull request builds
• Verify the step is skipped on Ubuntu and macOS builds
• Download artifacts from a successful build and verify package contents

🤖 Generated with Claude Code

[Copilot]

Pull Request Overview

This PR adds automated NuGet package creation and artifact uploading to the CI workflow. The changes ensure that NuGet packages are automatically packed on successful builds and made available for download, but only for non-PR builds on Windows to avoid unnecessary duplication across different OS builds.

Key Changes:

• Added a step to pack NuGet packages using dotnet pack
• Added a step to upload the packed packages as GitHub Actions artifacts with 30-day retention

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