Registry sample: retarget net10.0, MSTest 4.3.3, and honest CI for the Windows-only tests

[vladimir-pecanac-main]

Updates the ReadWriteWindowsRegistryInCSharp sample that backs Read and Write Windows Registry in C#.

Retarget

• Both projects net7.0 -> net10.0. Deliberately plain net10.0, not net10.0-windows: the two CA1416 warnings the article prints and explains only exist because the TFM is platform-neutral, and net10.0-windows would also break the Linux CI build.
• Test packages: Microsoft.NET.Test.Sdk 17.6.0 -> 18.9.0, MSTest.TestAdapter/MSTest.TestFramework 2.2.10 -> 4.3.3, coverlet.collector 3.2.0 -> 10.0.1.

Sample fixes

• Removed the redundant null-conditionals and the dead null-coalescing fallbacks. CreateSubKey(string) returns a non-nullable RegistryKey, so OpenSubKey(name, true) ?? CreateSubKey(name) is non-nullable too, and every subKey?. after it taught the reader the opposite of the truth.
• Array.Empty<string>() -> [].
• Stopped using Path.Combine() to build strings that are not paths: one builds a registry path, the other an NT account name. Both happened to work only because Path.Combine joins with \ on Windows. Replaced with explicit interpolation.
• OpenRemoteBaseKey(): the returned key was assigned to an unused variable and never disposed, and catch { } swallowed everything. Now using on the returned key, and the catch is narrowed to ArgumentException and IOException (an unreachable machine and a stopped Remote Registry service both surface as ArgumentException via Arg_RegKeyNoRemoteConnect).

Honest CI on a Windows-only sample
Every test here writes and deletes a subkey under HKEY_CURRENT_USER on the machine that runs it. They all used to open with if (!OperatingSystem.IsWindows()) return;, so the Linux runner reported 10 passed having asserted nothing.

• The early returns are gone and all ten tests are renamed to carry Live.
• This branch also carries the one-line workflow change from PR Build: exclude Live tests from the CI test run #2162 (dotnet test ... --filter "FullyQualifiedName!~Live"), identical line, merge-safe duplicate. A PR runs the workflow from its own branch, so without it here the renamed tests would run on the Linux runner and fail.

Verification

• dotnet build -c Release on SDK 10.0.302: 0 errors. 2 CA1416 warnings in the app project (the two the article shows), plus 4 CA1416 and 2 MSTEST0017 in the test project from the removed guards and the MSTest 4.x analyzers.
• dotnet test -c Release --no-build --filter "FullyQualifiedName!~Live": exit 0, no test matched, which is the honest result.
• The Live tests were not run in prep: they write to the real registry.