Unexpected type mismatch error for Either and Either.Right

This was initially reported in https://github.com/redhat-developer/vscode-java/issues/4178, but is actually an ECJ issue, tested on Eclipse 2025-06.

The following code, using Reactor Core's Mono and Arrow Core's Either generate a compilation error, while javac compiles fine:

```java
public Mono>> nok() {
return Mono.just(new Either.Right<>(Collections.emptyList()));
}
```

I'm attaching a self-contained Eclipse project, including the required jars, you can import it as a regular Eclipse project. It can also be imported as a Maven project, it makes no difference for JDT. FTR, I used my very very old [JBoss Tools JDT feature](https://docs.jboss.org/tools/whatsnew/core/core-news-3.3.0.M4.html) to convert Maven dependencies to local lib jars (menu now named "Copy Classpath Libraries...")

[testproject.zip](https://github.com/user-attachments/files/22737863/testproject.zip)

cc @jjohnstn