getJarClasses() throws NPE for missing version key in JarVersionedRuntimes

In `JarVersionedRuntimes.java:47-49`:

```java
public JarClasses getJarClasses(Integer version) {
return versionedRuntimeMap.get(version).getJarClasses(); // NPE if version absent
}
```

If the requested `version` is not a key in the map, `get(version)` returns `null`, then `.getJarClasses()` throws a NullPointerException. This is inconsistent with `getJarVersionedRuntime()` on the same class, which safely returns `null` for missing keys. Any caller querying a non-existent version will crash.

**Fix**: Guard against null, e.g.:
```java
JarVersionedRuntime runtime = versionedRuntimeMap.get(version);
return runtime != null ? runtime.getJarClasses() : null;
```