The JSON Schema advertised for tools renders optional fields as `X | null`
because JSON cannot express undefined, so callers (models, Code Mode agents)
legitimately pass null meaning "omit" and were rejected with
'Expected string | undefined'. Decode now retries with null-valued object
properties removed when the first attempt fails: schemas that genuinely accept
null succeed on the first attempt, array elements stay positional, and the
original error is reported when the retry cannot help.
Kit Langton
98310eb310