Strengthen preview/stable target-list test by asserting non-empty preview results

[Copilot]

This stacked PR follows up on review feedback from #335: the preview package list in FirmwarePackage_ListReferenceTargets was only checked for non-null, which could let the preview-vs-stable invariant silently pass when preview results are empty.

• Test correctness hardening

  • Added a non-empty assertion for the preview package list in nanoFirmwareFlasher.Tests/FirmwarePackageTests.cs.
  • This aligns the preview list checks with the existing pattern used for stable/platform-filtered lists.

• Behavioral impact

  • The test now fails fast when preview retrieval returns an empty set, instead of skipping the core preview-vs-stable containment check via a no-op loop.

Assert.IsNotNull(preview);
Assert.AreNotEqual(0, preview.Count);

[Copilot]

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• api.cloudsmith.io
  • Triggering command: /usr/share/dotnet/dotnet /usr/share/dotnet/dotnet exec --runtimeconfig /home/REDACTED/work/nanoFirmwareFlasher/nanoFirmwareFlasher/nanoFirmwareFlasher.Tests/bin/Debug/net8.0/nanoFirmwareFlasher.Tests.runtimeconfig.json --depsfile /home/REDACTED/work/nanoFirmwareFlasher/nanoFirmwareFlasher/nanoFirmwareFlasher.Tests/bin/Debug/net8.0/nanoFirmwareFlasher.Tests.deps.json /home/REDACTED/work/nanoFirmwareFlasher/nanoFirmwareFlasher/nanoFirmwareFlasher.Tests/bin/Debug/net8.0/testhost.dll --port 38871 --endpoint 127.0.0.1:038871 --role client --parentprocessid 4414 --telemetryoptedin false (dns block)
  • Triggering command: /usr/share/dotnet/dotnet /usr/share/dotnet/dotnet exec --runtimeconfig /home/REDACTED/work/nanoFirmwareFlasher/nanoFirmwareFlasher/nanoFirmwareFlasher.Tests/bin/Debug/net8.0/nanoFirmwareFlasher.Tests.runtimeconfig.json --depsfile /home/REDACTED/work/nanoFirmwareFlasher/nanoFirmwareFlasher/nanoFirmwareFlasher.Tests/bin/Debug/net8.0/nanoFirmwareFlasher.Tests.deps.json /home/REDACTED/work/nanoFirmwareFlasher/nanoFirmwareFlasher/nanoFirmwareFlasher.Tests/bin/Debug/net8.0/testhost.dll --port 42043 --endpoint 127.0.0.1:042043 --role client --parentprocessid 4502 --telemetryoptedin false (dns block)
• https://api.github.com/repos/nanoframework/nanoFirmwareFlasher/releases/latest
  • Triggering command: /usr/share/dotnet/dotnet /usr/share/dotnet/dotnet exec --runtimeconfig /home/REDACTED/work/nanoFirmwareFlasher/nanoFirmwareFlasher/nanoFirmwareFlasher.Tests/bin/Debug/net8.0/nanoFirmwareFlasher.Tests.runtimeconfig.json --depsfile /home/REDACTED/work/nanoFirmwareFlasher/nanoFirmwareFlasher/nanoFirmwareFlasher.Tests/bin/Debug/net8.0/nanoFirmwareFlasher.Tests.deps.json /home/REDACTED/work/nanoFirmwareFlasher/nanoFirmwareFlasher/nanoFirmwareFlasher.Tests/bin/Debug/net8.0/testhost.dll --port 38871 --endpoint 127.0.0.1:038871 --role client --parentprocessid 4414 --telemetryoptedin false (http block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)