An installed JDK that cannot run, such as an Intel-only JDK on Apple silicon, was registered as a project runtime and could crash Java language server initialization even with a working JDK selected. Each JDK's executable is now probed before selection, off the UI thread and outside the cache lock; unusable JDKs are left out, working Java 8 runtimes stay, and macOS bundle paths are accepted. An explicit choice that cannot run reports its actual executable. Since JDT keeps VM installs in its workspace, a project with an excluded JDK gets one fresh workspace; everyone else keeps their path.
Fixes#1668