Minimal JSON validator test: retain the failing token

To isolate a JSON validation failure, remove unrelated fields until the smallest safe input still produces the error. Preserve the exact quote, escape, trailing comma, duplicate key, or nesting that matters. State whether the validator is checking JSON syntax or a schema. If a tiny input passes after removing one field, inspect that field's bytes rather than guessing. Avoid posting private values; synthetic content can retain the same structure.