Update ECA to 3.0, including Modeler API and BPMN

I've updated the constraints. Also added Modeler API as an explicit requirement. However, that could be removed as it is required implicitly by BPMN_io and also by AI Agents. I leave that decision to the project lead to be made.

In a second step, I will also update the ECA models to the new format so that new installations will get them directly without having to be migrated afterwards.

Looks like we have a lot of failing tests which is due to schema errors with the existing models. That's actually expected and will be resolved when the models for updated as stated in #3

Merged into 2.x. Thanks! For caution's sake, not backporting this unless there's a strong need, which doesn't appear to be the case.

Automatically closed - issue fixed for 2 weeks with no activity.