Judge parser: a lone score object inside an unterminated array is accepted, inside an unterminated object it is not #1048

Summary

_parse_judge_json fails closed on a score object that follows an undecodable { (the object's extent is unknown, so the score object may be nested inside it). It does not do the same for an undecodable [, so a lone score object inside an unterminated array is accepted as the verdict, while the same object inside an unterminated object, or inside a terminated array, is not.

Raised in review of #1046 (outside its diff).

Reproduction (offline, development at 1877d93)

An object that sits in a container that was never closed cannot be shown to be the judge's own verdict, which is the reason the { rule exists. A reply cut off by the token cap is already rejected by #1045; this covers a reply that ends normally but is malformed.

Fix

Keep the rule for {. For an undecodable [, count a score object that follows as nested when the bracket is still open where the object starts (the number of [ minus the number of ] between the bracket and the object is above zero). Prose that closes its brackets before the verdict stays accepted: See [1] and [source](https://example.org): {"score": 2} and As noted [1] the answer is fine. {"score": 7} (both are pinned by tests today).

Acceptance criteria

• The first two rows above return None; every reply the existing parser tests accept is still accepted.
• Regression tests: the rows above, a nested unterminated array ([ [x] , {...}), brackets that close before the verdict, and a stray unclosed [ in prose before a verdict (documented as failing closed).
• A ### Fixed entry under ## [Unreleased] in CHANGELOG.md.

Context

Related: #127, #1038, #1045, #1046.