Add ST_IsValid and ST_MakeValid for geography

## Description

Follow-up to #1085 and #1143.

`ST_GeogFromWKB`/`ST_GeogFromWKT` currently accept rings whose edges cross when interpreted as geodesics, and geography behaviour for such input is undefined. #1143 makes the default ring-orientation heuristic less surprising for that kind of input, but the principled path is to detect and repair it:

- `ST_IsValid(geography)` — report whether the input is a valid spherical geography (e.g., no crossing edges), so pipelines can find problem geometries like the Overture division boundary in #1085 (its planar interpretation is valid; only the geodesic interpretation self-intersects).
- `ST_MakeValid(geography)` — rebuild the input into a valid geography (e.g., via an S2Builder snap/repair pass), so joins and measures are well defined afterwards.

For context, the BigQuery comparison checks already had to work around this class of input by using a "make valid" WKB constructor because at least one polygon in the test corpus fails validation with a crossing-edges error (see discussion in #1085).

Start by tracing the existing geography function entry points and the BigQuery comparison checks referenced in #1085. Investigate how an S2Builder snap/repair pass could support ST_IsValid and ST_MakeValid. Done means crossing-edge geographies can be detected and repaired so subsequent joins and measures have defined behavior.

Written by the indexing model from the issue text.