fix: parse single-line fenced json responses

[pollychen-lab]

Summary

• allow fenced JSON extraction when the opening fence and JSON payload are on the same line
• add a regression test that proves the fenced-block fast path wins over malformed preamble braces

Fixes #97

Validation

• python3 -m pytest tests/test_llm_backend.py -q
• . .venv/bin/activate && python -m pytest -q
• . .venv/bin/activate && ruff check src/ tests/
• git diff --check
• rg -n 'Karim13014|claude-code@anthropic.com' . -g '!.git' -g '!.venv'

[alphacrack]

Thanks — the single-line fence case from #97 is real, and your regression test correctly fails on main (verified; full suite 282 passed locally at this head). One regression to address before merge, though:

The widened regex now captures non-JSON fences. With the \n requirement gone and (?:json)? matching empty, any fenced block — or inline triple-backtick span — takes the fence fast path, swallowing the language tag into the payload. Concretely:

Setup:
```bash
pip install x

{"feasible": true}

main finds no JSON fence here and the brace-scan fallback returns the JSON; this branch extracts `bash\npip install x`, `json.loads` fails, and `complete_json` burns its retries on every such planner/distiller response — or exhausts them and hard-fails the stage.

Suggested guard: take the fence path only when the payload parses as JSON (fall through to the brace scanner otherwise), or require the `json` tag when the opener has no trailing newline.

Also worth noting: this is the repo's only `extract_json` test, and the PR changes the function's only regex. Could you add cases pinning the previously-working shapes — multi-line ```json fence, bare ``` fence, the no-fence brace fallback, and a ```bash fence followed by raw JSON? That last one exposes the regression above.