[FrankYang0529]

Implement Go SDK: Add dag.Switch with Case.

• dag.Switch(fn, opts...) and TaskGroupRef.Switch add a task that runs the decider function fn, and return an *airflow.SwitchRef. fn returns (*airflow.TaskRef, error).
• SwitchRef.Case(task) names a task that the decider can choose and records the edge from the decider to that task. Case returns the switch, so one statement can name every case: dag.Switch(pickPath).Case(long).Case(short).
• When the decider returns a case, the decider task pushes the task_id of that case as its own return_value XCom. The decider task then writes the task_ids of the other cases to the skipmixin_key XCom and skips those cases.
• When the decider returns a *airflow.TaskRef that is not a case, the decider task fails and pushes no XCom.

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

• Yes - Claude Code

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[jason810496]

Thanks for the review, LGTM overall.
Here're some comments from the claude review.

[jason810496]

Thanks.