JUnit 4 test case is launched with JUnit 6 so no tests are ran

I'm trying to run this test case: `org.eclipse.jdt.debug.tests.ui.OpenFromClipboardTests`

It has many JUnit 4 annotations:

```
import org.junit.After;
import org.junit.AfterClass;
import org.junit.Assert;
import org.junit.Before;
import org.junit.BeforeClass;
import org.junit.Rule;
import org.junit.Test;
```

When I try to run it, the JUnit 6 launch type is used. But we decided to not include the vintage engine, since its deprecated in JUnit 6. So the launch finds no tests:

```
null
org.eclipse.jdt.junit
Error
Thu Mar 05 14:06:27 EET 2026
No tests found with test runner 'JUnit 6'.
```

So for this case we will have to explicitly set JUnit 4 for the launch type, otherwise no tests are ran. I see this both with the plain JUnit test launch and the JUnit plug-in test launch. Hopefully the code is only in JDT UI.

Start with org.eclipse.jdt.debug.tests.ui.OpenFromClipboardTests and trace how the plain JUnit test and JUnit plug-in test launch types select their runner. Reproduce the launch with the JUnit 4-annotated test, then verify that both launch paths choose JUnit 4 and discover the tests instead of reporting no tests found.

Written by the indexing model from the issue text.