Rollup: support segment-partitioned sources as the expected set

[Lee-W]

Body

What

Extend RollupMapper so its expected-set source can be a segment set (from #57695) in place of a time-based window. Completeness semantics are the same; only the source of the expected set differs. Example: a global report fires after all regional partitions arrive.

Committer

• I acknowledge that I am a maintainer/committer of the Apache Airflow project.