Segment: completeness semantics against a declared set

### Body

### What
Define what "all partitions present" means when the partition set is categorical. Temporal completeness is inferable from a window (31 days in March); segment completeness needs the declared or runtime-resolved set as the reference.

### Related
#57695, #44146, #59294

### Committer

- [x] I acknowledge that I am a maintainer/committer of the Apache Airflow project.