AIP-76: Propagate partition_date to consumers of partitioned assets

[nathadfield]

What this does

Makes partition_date a first-class template variable on the consumer side of AIP-76 partitioned assets, so authors can write:

WHERE dt = "{{ partition_date | ds }}"

instead of slicing strings out of partition_key.

How

• AssetEvent.partition_date is a new column (migration 0116); the producer stamps it from ti.dag_run.partition_date at emission, so the partition's datetime form rides every event the consumer sees.
• The consumer's target date is computed alongside target_partition_key in assets/manager.py:_queue_partitioned_dags and frozen on a new AssetPartitionDagRun.partition_date column. The scheduler simply copies that value into the consumer DagRun, so the date stays consistent with the mapper that produced the key even if mapper code or config is later edited.
• IdentityMapper passes the source date through; the StartOf*Mapper family normalises via a new internal to_downstream_normalized (no public mapper-protocol change). Other mappers (ProductMapper, ChainMapper, AllowedKeyMapper, custom) leave partition_date=None and consumers fall back to partition_key.
• partition_date is output-only on trigger payloads, so a manual trigger cannot create an inconsistent (partition_key, partition_date) pair.
• Surfaced through to Context["partition_date"] (coerced to tz-aware), the execution-API DagRun / AssetEventResponse / DagRunAssetReference (Cadwyn-versioned to strip the field for older Task SDK clients), the core-API DAGRunResponse, OTel span attributes, and the standard provider's PythonVirtualenvOperator / ExternalPythonOperator serializable-context allow-list.

Known limitations / follow-ups

• For ProductMapper, ChainMapper, AllowedKeyMapper, and custom mappers, partition_date is None. A public to_downstream_partition_date extension hook would close that gap but expands the mapper protocol — intentionally deferred.
• If two source events resolve to the same target_partition_key with different normalised datetimes (e.g., two upstream assets with different timezone-configured mappers), the first event wins and a warning is logged in _get_or_create_apdr.

closes: #67239

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