Use Process.StartAndForget for fire-and-forget process launches

[Copilot]

Adopt Process.StartAndForget (dotnet/runtime#126078) at call sites where Process.Start was used in a fire-and-forget pattern, leaking the returned Process handle.

• CleanFileBasedAppArtifactsCommand.cs — cleanup subprocess launched with discarded return value
• ServerConnection.cs — Razor server subprocess launched, only .Id read from the handle

Both sites set RedirectStandardInput/Output/Error but never consumed the streams; these flags are removed since StartAndForget redirects to the null device by default and rejects explicit redirect flags.

- Process.Start(startInfo);
+ _ = Process.StartAndForget(startInfo);

- var process = Process.Start(startInfo);
- processId = process.Id;
+ processId = Process.StartAndForget(startInfo);

Note: Will not compile until MicrosoftNETCoreAppRefPackageVersion is updated to a build containing Process.StartAndForget.

[adamsitnik]

to other reviewers: the idea of using Process.StartAndForget is to avoid resource leaks, but in this case redirecting standard out/err and NOT consuming them can simply lead to deadlocks: the started process can fill the pipe buffer and will just wait for the data to be consumed on some Console.Write* method. And here nothing was consuming the produced data.

If it was done to prevent from printing the produced output to terminal then the new method takes care of that as well, as it by default redirects to NUL device (unless configured explicitly)