#2467 [Bug]: json_object silently discards elements from array-root output

Description

### What happened

With `response_format: {"type":"json_object"}`, an array-root model response can be accepted as a successful JSON object. `_extract_json()` selects the first balanced `{...}` inside the array and drops the remaining elements.

Reproduced on unmodified `main` (`296b59d`):

```bash
python3 - <<'PY'
from serve.structured import _ObjectOnly, validated_json
raw = '[{"answer":1},{"answer":2}]'
print(validated_json(raw, _ObjectOnly(), "stop"))
PY
```

Actual: `{"answer":1}`. The mock HTTP endpoint also returned 200 with that truncated object.

Expected: reject an array-root response with `structured_output_failed` (HTTP 502 for non-streaming requests), not silently reinterpret it as an object. Existing support for an object inside prose or a Markdown code fence (#762) should remain intact.

### Strata version, GPU, OS

`main` at `296b59d` · Ubuntu 24.04 LTS · Python 3.12 · GPU not used (parser-only and MockEngine HTTP reproduction).