Structured Output Parser

Task

This module pulls one JSON object out of a model reply and checks it against a schema. It fails in production now and then. You have forty real replies and two tests. Write tests that pin down what the parser must do, then fix the parser. Your tests are graded too: each one must state the exact result, and the suite must fail on wrong versions of the parser.

Requirements

1. check(request) is the only function the checks call. {op: "parse", reply, schema} returns parseReply(reply, schema). reply must be a string (Error containing reply). Any other op raises an Error containing op. Do not change the arguments.
2. Finding the object. If the reply has fenced blocks, the object comes from the first fenced block that holds one. Otherwise it comes from the prose. The answer is the first balanced {...} span, reading left to right, that parses as a JSON object. Braces inside JSON strings do not count. When nothing parses, return {ok: false, reason: "no_json"}.
3. Repair. Before parsing, a comma that is followed only by white space and then } or ] is removed. Text inside JSON strings is never changed.
4. Validation. A schema is a list of {name, type, required}; type is string, number, boolean, array or object. A valid object returns {ok: true, value}, with the value exactly as parsed and extra keys kept. Otherwise return {ok: false, reason: "invalid", errors}, where errors lists {field, problem} in schema order. problem is "missing" for an absent required field and "wrong_type" for a value of another type. Values are never converted, and null is a wrong type.
5. Tests. TESTS holds {name, reply, schema, expected}. {op: "suite"} returns {failed}: the names of tests that fail on your parser. {op: "catches", variant} returns {caught}: true when a test that passes on your parser fails on that variant (Error containing variant when unknown). {op: "covers", outcome} returns {covered}: true when a passing test expects that outcome ("ok", or a reason).