Java SDK: native Dag definition

[jason810496]

Body

We should wait until AIP-85 Dag Importers pass.

Background

The Java SDK (1.0.0-beta1) intentionally ships in "Python-stub-Dag mode": Java SDK handlers are registered for Dag/task IDs declared in Python. Higher-level Java Dag authoring was deferred until coordinator-mode execution is reviewed.

What needs to happen

1. Allow declaring Dags natively in Java-SDK.
2. Revisit bundle ergonomics alongside the authoring interface.

Acceptance criteria

• A Dag can be authored end to end in Java with no Python stub file.

Committer

• I acknowledge that I am a maintainer/committer of the Apache Airflow project.