TypeScript SDK: native TypeScript Dag declaration

[jason810496]

We should wait until AIP-85 Dag Importers pass.

Background

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

What needs to happen

1. Allow declaring Dags natively in TypeScript.
2. Revisit bundle ergonomics alongside the authoring interface.

Acceptance criteria

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

Context

• Add TypeScript task SDK public interface #67908 (PR description)
• Add TypeScript coordinator runtime #69079 (PR description)