Validate a JSON value against a JSON Schema (draft 2020-12 default; 2019-09/7/4 via $schema). Send {data, schema}; returns {valid, errors:[{path, message, keyword}]} with a JSON Pointer path per failure. For agents checking structured/tool-call output before use. Runs on an eval-less validator (Ajv can't run on Workers). Remote $ref is refused (self-contained only); unsafe regex patterns are rejected before compilation.