TS SDK: add multi-way branching with dag.switch

[jason810496]

TS SDK: add multi-way branching with dag.switch

• Depends on TS SDK: add conditional branching with dag.if #73443 get merged first (only the last commit is new here).
• Diff for early review: jason810496/airflow@feature/ts-sdk/conditional-branching...feature/ts-sdk/multi-way-branching
• next: TS SDK: add triggerDagRun to the native Dag surface #73445
• 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

The multi-way form of the same ADR: TypeScript's switch/case.

How

const decider = dag.task("pick_path", async ({ rows }: { rows: number }) =>
  rows > 1000 ? handleLong : handleShort,
);
const picked = decider({ rows: extracted });

dag.switch(picked).case(handleLong).case(handleShort);

• Both the decider and each case are task references the Dag already handed back, per decision 2, so the compiler checks the candidate exists and renaming a handler cannot silently rewire a Dag.
• The task's value on the wire is the chosen task's id, which is what a downstream task reads back from its XCom.
• No default case, per decision 3: a decider returning anything outside its cases fails the task, naming what it chose and what it could have chosen. A branch with no case at all decides nothing and is rejected when the Dag is read.
• Exactly one case is selected — the limitation that ADR records for every language SDK.
• As with dag.if, the decider is serialized with _can_skip_downstream and writes the skipmixin_key XCom, so a cleared case stays skipped.

What

• dag.switch and the chainable Branch. Unlike a condition's then, case is not a thenable trap, so nothing needs guarding here.

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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

[jason810496]

Finished in #74070.