Update NuGet audit CVE handling

[boblangley]

Summary

• Replicate the sandboxed NuGet audit workflow behavior from InternalAutomationSandbox
• Capture dotnet restore output to a log, count NU1901-NU1904 warnings with awk, and expose the count as a cvecount step output
• Gate result upload and artifact archival on the cvecount output instead of the restore step failure state
• Update the NuGet audit documentation to describe the new CVE detection flow

Validation

• Parsed .github/workflows/nuget-audit.yml with Ruby YAML
• Ran git diff --check with CRLF-aware whitespace settings

[tamararivera]

I take this https://github.com/Particular/InternalAutomationSandbox/actions/runs/26898126213 as proof that it is working as expected.