Typed error when a query requires spatial but the extension is unavailable #16

Motivation

querychat's "maps" capability (posit-dev/querychat#323) needs to give the LLM actionable guidance when a ggsql query requires spatial features but the DuckDB spatial extension isn't available (never installed, or setup failed). Today querychat string-matches DuckDB error messages ("spatial" + "extension", or st_* catalog errors), which is fragile across DuckDB versions and only catches failures at execution time.

Proposal

Raise a dedicated exception type (e.g. SpatialExtensionError) when a query uses spatial features (DRAW spatial, ggsql:world, PROJECT TO <map>) and the spatial extension is unavailable — ideally at validate() time, since querychat already calls validate() before execute(), which would let callers gate on it without ever hitting DuckDB.

Callers can then catch a type instead of matching strings.