Support Convex polygons for polygon search

Currently, convex polygons will fail when performing a polygon search. This is currently due to `healpy` and `lsst-sphgeom` not supporting convex polygons. One approach could be to split a convex polygon into concave polygons, for example [something like this](https://sseemayer.github.io/Py2D/documentation/features/convex_decompose.html). Or to look for other/contribute to dependencies.

Start by tracing the polygon-search path and reproducing the failure with a convex polygon. Read the integration points with healpy and lsst-sphgeom, then determine whether decomposition or dependency changes can support the query. Done means convex polygon searches complete successfully with appropriate coverage and regression tests.

Written by the indexing model from the issue text.