ENH: siav2 implementation doesn't allow "contain coordinate" queries

The first use case in the SIAv2 document is a simple contain or not question

> find data that includes specified coordinates (e.g. for some object)

Yet, with POS it's not possible to query for this, one always have to pass on a search radius or define a range/polygon.

Would you think there could be/should be a way to work around this? E.g. in pyvo could default to very small radius when only a skycoord is passed along for a coordinate?

The issue does not name files or tests. Start by tracing pyvo's SIAv2 POS handling and the query path used when a SkyCoord is passed without a radius, then compare it with the SIAv2 containment use case. Done requires an agreed query behavior and corresponding coverage for coordinate-only containment queries.

Written by the indexing model from the issue text.