AIP-76: Add PartitionAtRuntime authoring API to Task SDK

[anishgirianish]

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First of three PRs decomposing draft #65300 for AIP-76, per the design agreed in #44146.

Adds the Task SDK producer surface for runtime partition keys:

• PartitionAtRuntime() marker timetable
• PartitionKey(key, extra) for per-event metadata
• outlet_events[asset].partition_keys = [...] / .add_partition(...)
• self.partition_keys = [...] inside @asset via _AssetSelfProxy
• outlet_events kwarg injection in @asset / @asset.multi

No server-side consumption yet, that lands in PR 2 (core behavior + fan-out) along with PR 3 (example DAG + docs).

cc: @Lee-W

related: #44146
related: #65300

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[Lee-W]

I think we also need airflow-core counter part for PartitionAtRuntime

[potiuk]

@anishgirianish Converting to draft — this PR doesn't yet meet our Pull Request quality criteria.

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[potiuk]

Quick follow-up to the triage comment above — one clarification on the "Unresolved review comments" item:

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Note: This comment was drafted by an AI-assisted triage tool and may contain mistakes. Once you have addressed the points above, an Apache Airflow maintainer — a real person — will take the next look at your PR. We use this two-stage triage process so that our maintainers' limited time is spent where it matters most: the conversation with you.

[anishgirianish]

Quick follow-up to the triage comment above — one clarification on the "Unresolved review comments" item:

Once you believe a thread has been addressed — whether by pushing a fix, or by replying in-thread with an explanation of why the suggestion doesn't apply — please mark the thread as resolved yourself by clicking the "Resolve conversation" button at the bottom of each thread. Reviewers don't auto-close their own threads, so an addressed-but-unresolved thread reads as "still waiting on the author" and keeps the PR from moving forward. The author doing the resolve-click is the expected convention on this project.

Sure, thank you.

[anishgirianish]

I think we also need airflow-core counter part for PartitionAtRuntime

Done, added PartitionAtRuntime(NullTimetable) in airflow/timetables/simple.py and registered it in serialization/encoders.py.

Thank you

[anishgirianish]

@Lee-W Thank you so much for the review. addressed all three comments, ready for another look whenever you get a chance, thanks.

[Lee-W]

If I'm understanding it correctly, we're only adding the interface but not actually making it work yet? (since we did not touch the code in scheduler and use partitioned_at_runtime)

[anishgirianish]

If I'm understanding it correctly, we're only adding the interface but not actually making it work yet? (since we did not touch the code in scheduler and use partitioned_at_runtime)

@Lee-W Thank you much for the review! Yep this PR is interface only; the scheduler wiring + example DAG + docs will land together in a follow-up PR. I have addressed feebacks in the latest push.Would like to request you for a another look when you whenever you get a chance?

[Lee-W]

mostly good from my end and would need @uranusjr 's help for another round.

also please take a look at airflow-core/src/airflow/api_fastapi/execution_api/versions/v2026_04_06.py and contributing-docs/19_execution_api_versioning.rst. it's the last missing piece from me :)