[FrankYang0529]

Implement Go SDK: Add airflow.Dag and dag.Task, the native Dag constructors.

• airflow.Dag(dagId, spec) returns a *DagRef, which is a Registerable. Register stores the Dag separately from the task handlers and marks it as registered, so a later call to dag.Task panics.
• dag.Task(fn, opts...) runs the same signature check as TaskHandler and returns a *TaskRef. The task_id is the Go function name, spelled exactly as in the source, unless TaskSpec{TaskID: ...} sets one.
• Verification: go test ./...

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

• Yes - Claude Code

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• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.

[jason810496]

Thanks for the PR, LGTM overall.

[jason810496]

One minor nit, then I think we're good to go, thanks.

[jason810496]

Thanks.