Expand tasks in mapped group at parse time

[uranusjr]

Depends on #27027. Merged; rebased to main.

This involves a few things:

• The mapped task group needs to implement parse-time and run-time ti count accessors, similar to the mapped operator.
• Since a non-mapped operator can be in a mapped task group and thus needs to be expanded, the parse-time and run-time ti count accessors are "raised" from MappedOperator to AbstractOperator (so they can be used by BaseOperator). These now also considers an operator's parent task groups.
• If a ti count accessor is called on a non-mapped operator that's not in any mapped task group, the accessor should return a special value of a sort. Since None is already used, we raise an exception in this case instead (simply called NotMapped). For consistency, the previous case where None was returned (upstreams not available) is also changed to raise an exception instead (reusing NotFullyPopulated). This means the ti count accessors now always return an int, and raise an exception whenever the count is not available.
• All of the above result in a coherant interface in AbstractOperator that can be used in DagRun to perform ti count revision.

[uranusjr]

I think this is ready, hopefully CI agrees. (Edit: it does.)