TS SDK: add task groups to the native Dag surface

[jason810496]

TS SDK: add task groups to the native Dag surface

• TS SDK: draw order-only edges with before and after #73438 is merged, so this one applies to main on its own.
• Diff for early review: main...jason810496:airflow:feature/ts-sdk/task-groups
• next: TS SDK: serialize a native Dag to Dag JSON #73441
• Part of the native TypeScript Dag stack, TS SDK: declare task dependencies by calling tasks #73435 to TS SDK: cover native TypeScript Dags end to end #73446, to be merged in order.

Why

ts-sdk/adr/0002 decision 5 keeps Python's grouping semantics: a scope offering the same task and taskGroup methods as the Dag, prefixing each task id, and standing at either end of an edge as group1 >> group2 does.

How

• dag.taskGroup(groupId, options?) opens that scope and prefixes the id of everything declared in it, as prefix_group_id does. { prefixGroupId: false } keeps the ids as written.
• A group id is checked as Python's validate_group_key does: at most 200 characters, letters, digits, dashes and underscores only.
• Node is the base type both a task and a group satisfy, so before and after take either. Python has the same in DAGNode; the Go SDK calls it airflow.Node.
• Tasks and groups share one id namespace, as they do in Python. The separator cannot appear in an id of either kind: a dotted task id used to be accepted verbatim, giving a task whose id carries a prefix while the Dag holds no such group.

const staging = dag.taskGroup("staging");
staging.task("stage_rows", stageRows)(); // task id "staging.stage_rows"

const checks = dag.taskGroup("checks", { prefixGroupId: false });
checks.task("nulls", checkNulls)(); // task id "nulls"

What

• TaskGroupRef, TaskGroupOptions, Node, and TaskGroupRecord as what the Dag records.
• An edge endpoint is checked by identity for a group as it already was for a task; matching on the id alone accepted a group from another Dag object and retargeted the edge locally.

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Was generative AI tooling used to co-author this PR?

• Yes, with help of Claude Code Opus 5 following the guidelines

[pierrejeambrun]

Two small nits, overall looking good.

[jason810496]

Thanks for the review.

[pierrejeambrun]

LGTM, thanks