The schema is checked before any work (Decider._check_schema): a missing schema, a non-object schema or field, choice options
that are not a non-empty list of strings, a scale legend that is empty or has non-finite keys or values, and an unknown type
are 422s whose message names the expected form. Every schema 1.1.2 answered is answered the same way, except options or a
legend given as a bare string (1.1.2 split it into characters) and non-string options with neutralize_none off. An empty
schema returns {} without a forward pass. /v1/systemone with an empty question map still returns 200. 119 tests.

Fixes #8

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>