[AIP-94] Mark tasks CLI commands as migrated to airflowctl

[YAshhh29]

Add the @deprecated_for_airflowctl marker decorator to the 5 tasks CLI
commands that have airflowctl equivalents:

• tasks failed-deps
• tasks state
• tasks list
• tasks states-for-dag-run
• tasks clear

This follows the simplified marker-only direction described in #68402
(comment by @bugraoz93) and the same pattern as #68932 (variables) and
#68958 (config).

related: #68402

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[bugraoz93]

Thrse still need parity in airflowctl. Only a couple left but we should wait those for tasks. Thanks for the PR

[bugraoz93]

https://cwiki.apache.org/confluence/spaces/AIRFLOW/pages/382175838/AIP-94+Decouple+Remote+Commands+from+airflow+CLI+to+airflowctl#AIP94DecoupleRemoteCommandsfromairflowCLI(toairflowctl)-Whatchangedoyouproposetomake%3F

[YAshhh29]

Thanks for the review and the context! That makes sense — I'll keep this PR open so it's ready to merge as soon as the remaining [airflowctl tasks] commands land and parity is complete. Happy to help implement any of those if it'd be useful.

[potiuk]

Thanks — marker-only change, and the decorator ordering matches the existing task_states_for_dag_run / task_clear entries, with the registry test updated alongside. Straightforward.

One process question, not a blocker since deprecated_for_airflowctl is explicitly maintainer-facing and renders nothing to users: of the three targets named here, only states-for-dag-run is currently in TASK_COMMANDS — airflowctl tasks failed-deps, tasks state and tasks list don't exist on main yet (they're #69397, #70276 and #70472, all still open). tasks clear is already marked the same way, so this is clearly the established batching pattern rather than something new here.

Is the intent that the marker leads the implementation and the registry records "claimed for airflowctl" rather than "available in airflowctl"? If so this is fine as-is; if the registry is meant to reflect what actually ships, it might be worth landing these markers behind their respective command PRs. Either way I'm happy for it to go in — just want the convention written down somewhere if it isn't already.

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Drafted-by: Claude Code (Opus 5); reviewed by @potiuk before posting