Optionally emit Dag tags as structured log context on task logs

[hanxdatadog]

Optionally emit Dag tags as structured log context (logging counterpart to dag_tags_in_metrics)

What does this PR do?

• Adds a new [logging] dag_tags_in_logs config option (boolean, default False).
• When enabled, task_runner.startup() binds Dag-tag-derived fields into structlog contextvars right after the Dag is parsed, so every subsequent log line for that task attempt carries them.
• Reuses build_dag_metric_tags() (the same helper backing [metrics] dag_tags_in_metrics, added in Optionally emit Dag tags as metric tags #68568) so tags of the form key:value split into a key/value field; plain tags map to an empty-string value — keeping the same tag-parsing convention across metrics and logs.
• Built-in TI identifier keys (ti_id, dag_id, task_id, run_id, try_number, map_index) always win on collision with a same-named Dag tag.

Motivation

Dag tags (e.g. team:data-eng, env:prod) are already surfaced in metrics via dag_tags_in_metrics, but not in structured logs — users who want this today have to write a custom Airflow listener plugin to bind the context themselves. This closes that gap with a small, opt-in, native option, mirroring the metrics precedent.

Testing

• Unit tests: 4 new tests added to task-sdk/tests/task_sdk/execution_time/test_task_runner.py covering: flag off (no leak into log context), flag on with mixed key:value/bare tags, flag on with no tags, and the reserved-key collision guard.
• Ran via breeze testing core-tests: 7/7 pass (4 new + 3 pre-existing dag_tags_in_metrics/stats_tags tests, confirming no regression).
• Static checks: ruff format, ruff check, mypy-task-sdk, YAML lint, newsfragment validation — all pass via prek.

Additional Notes

• Intentionally scoped to task_runner.py + config + tests only — no changes to the Listener Plugin API, executors, or Celery. Dag tags are already available in-process at this point (the Dag is parsed just above), so no new DB/ORM access is introduced.
• Newsfragment filename: 71793.feature.rst.
• Related: Propagate DAG tags to Sentry for alert routing #37901 (Dag tags → observability tooling, currently Sentry-scoped) and Optionally emit Dag tags as metric tags #68568 (the metrics counterpart of this change).

Who will it impact?

Opt-in only (default False) — no behavior change for existing deployments unless explicitly enabled.

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

• Yes (Claude Code)