test(desktop): fail WinForms tests on UI-callback exceptions per STA thread

TestAssemblySetup set the unhandled-exception mode and a ThreadException
handler from a module initializer. WinForms stores both per thread
(ThreadContext/NativeWindow thread statics), so they applied only to the
runner thread that loaded the assembly, never to the STA threads the
tests run on. There, an exception escaping a window message or a
BeginInvoke/SynchronizationContext callback still opened a modal
ThreadExceptionDialog in an interactive session, or was silently ignored
otherwise; the handler would also have logged and swallowed it.

Replace the fifteen copied RunSta helpers with StaTestThread.Run, which
selects UnhandledExceptionMode.ThrowException on each STA thread before
it creates a window, so the exception propagates to the test body and
fails the test. Each class keeps its own timeout and message; all
threads are now background threads. Remove TestAssemblySetup.

StaTestThreadTests covers a throwing BeginInvoke callback through
StaTestThread (fails with the original exception) and, as a control,
the default mode on a plain STA thread (the exception goes to the
thread's handler and never reaches the test body). Cross-compiled on
Linux; Windows execution is the windows-desktop CI job.

OpenNest.WinForms.Tests/Controls/ProgramEditorControlTests.cs

@@ -1,6 +1,5 @@
 using System.Drawing;
 using System.Reflection;
-using System.Runtime.ExceptionServices;
 using System.Text.RegularExpressions;
 using System.Windows.Forms;
 using OpenNest.CNC;
@@ -292,19 +291,6 @@ public class ProgramEditorControlTests
     private static void ApplyHighlighting(ProgramEditorControl control) => typeof(ProgramEditorControl)
         .GetMethod("ApplyHighlighting", BindingFlags.Instance | BindingFlags.NonPublic)!.Invoke(control, null);
 
-    private static void RunSta(Action action)
-    {
-        Exception? error = null;
-        var thread = new Thread(() =>
-        {
-            try { action(); }
-            catch (Exception ex) { error = ex; }
-        })
-        { IsBackground = true };
-        thread.SetApartmentState(ApartmentState.STA);
-        thread.Start();
-        Assert.True(thread.Join(TimeSpan.FromSeconds(30)), "The STA test did not complete.");
-        if (error != null)
-            ExceptionDispatchInfo.Capture(error).Throw();
-    }
+    private static void RunSta(Action action) =>
+        StaTestThread.Run(action, TimeSpan.FromSeconds(30), "The STA test did not complete.");
 }