Limited SIA radius?

I don't find any such limit in the spec (https://www.ivoa.net/documents/SIA/20151223/REC-SIA-2.0-20151223.html), is this really necessary?
(My naive use case was to see if a collection had any data in it, so wanted to do as big of a search as possible, very naive approach, but still, I suppose (almost) all sky queries should be allowed at the library level?

```
334 if radius <= 0 * u.deg or radius.to(u.deg) > 90 * u.deg:
```

cc @msdemlei

Start at the radius validation shown around line 334 and compare its upper-bound behavior with the linked SIA 2.0 specification. Determine whether all-sky queries should be accepted at the library level, then identify or add coverage for zero, 90-degree, and larger-radius inputs; done means the behavior matches the agreed interpretation of the specification.

Written by the indexing model from the issue text.