jextract: Use fully qualified java.lang.String to avoid shadowing

[sidepelican]

When generating Java code for next Swift type a nested Java record named String is created.

public enum MyEnum {
    ...
    case string(String)
}

public final class JSON implements JNISwiftInstance {
  public record String(java.lang.String arg0) implements Case {
    record _NativeParameters(java.lang.String arg0) {}
  }

This causes the standard java.lang.String to be shadowed within that scope, leading to uncompilable source code where the generator expects the standard JDK String but finds the newly generated one instead.

To prevent this, the generator now uses the fully qualified name.

Note

This applies to other types as well (such as AtomicBoolean), but I am only addressing String in this PR for now.

[ktoso]

Lgtm!