Validate data against a canonicalized schema. Mirrors the
schema_expect semantics — returns the (possibly defaulted) value
on success and a thrown error string on failure.
Canonicalize a JSON-Schema-shaped value for use as an MCP elicitation
requestedSchema. Reuses the same canonicalizer the rest of the
language uses for schema_from_json_schema(...) so behavior is
identical between user-facing schema builtins and elicitation.