Add deadline and callback exports to airflow.sdk type stub

[rapsealk]

AsyncCallback, SyncCallback, DeadlineAlert and DeadlineReference are importable from airflow.sdk at runtime via its lazy import map, but task-sdk/src/airflow/sdk/__init__.pyi had no entries for them. The Deadline Alerts documentation tells users to write from airflow.sdk import DeadlineAlert, which then fails a type check:

error: Module "airflow.sdk" has no attribute "DeadlineAlert"  [attr-defined]

This adds the four names to the stub's imports and __all__. SyncCallback is included for stub/runtime parity — it is a public class in airflow.sdk.definitions.callback alongside AsyncCallback, and leaving it out would just reproduce the same gap for the next user.

Split out of #66269 at review request — that PR mixed this fix with an example Dag and a serialization fix.

Verification

• prek run mypy-task-sdk --all-files passes.
• With this change applied, a Dag using from airflow.sdk import DAG, AsyncCallback, DeadlineAlert, DeadlineReference, task typechecks clean under mypy-airflow-core; without it, mypy reports three attr-defined errors.

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

• Yes — Claude Code (Opus 5)

Generated-by: Claude Code (Opus 5) following the guidelines

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Important

🛠️ Maintainer triage note for @rapsealk · by @potiuk · 2026-08-13 12:55 UTC

Helpful heads-up from the maintainers — please address before this PR can be reviewed:

• ❌ Pre-commit / static checks. See docs.

Full list of what we check: Pull Request quality criteria.

The ball is in your court — you've been assigned to this PR. Fix the above, then mark it Ready for review.

_{Automated triage — may be imperfect; a maintainer takes the next look.}

[ferruzzi]

Closing this asit appears this has already landed. To be clear, your change was right, we just mixed up the merge order.

This was split out of #66269 at review request, but #66269 then merged on 29 August still carrying the same six lines in task-sdk/src/airflow/sdk/__init__.pyi, so the split never got reflected in the parent. I compared the two changesets line by line and they are identical. All six are on main today, each exactly once:

• from airflow.sdk.definitions.callback import AsyncCallback, SyncCallback
• from airflow.sdk.definitions.deadline import DeadlineAlert, DeadlineReference
• "AsyncCallback", "SyncCallback", "DeadlineAlert" and "DeadlineReference" in __all__

So the gap you reported is closed, including SyncCallback which you argued for on stub/runtime parity grounds, and the fix is credited to you via #66269 either way.

Thanks for doing the split, and apologies that it ended up unused. If you (or anyone else) sees this and thinks it's a mistake, please let me know.