fix: pass through a response that is JSON but not an object

[rickstaa]

Closes #64.

call_runner picks its return path from Content-Type alone: JSON parses into result.data, anything else arrives unparsed in result.content. A body that is valid JSON but not an object matched neither. It took the JSON branch, failed isinstance(data, dict), and raised Live runner call expected JSON object, got list.

A runner that proxies somebody else's API does not choose its response shape, and a top-level array is a common one — every Hugging Face inference task except text-to-image answers with one. Such a runner could not be called through the SDK at all. The workaround was for the runner to stop advertising JSON so the body took the opaque-bytes path, which in runner-app-examples#82 meant nginx relabelling a JSON body as text/plain.

What changed

Non-object JSON is now handed back unparsed in result.content, exactly as an image or ndjson already is, with result.content_type left saying application/json so the caller knows what it is holding.

Objects keep today's behavior including session_id extraction, so the only path that behaves differently is the one that used to raise. Strictness stays where the body carries protocol fields: proxy create and trickle channel remove are untouched.

test_json_array_still_rejected becomes test_json_array_returns_raw, and a scalar case joins it, since the rule is about being an object rather than about arrays.

Testing

• uv run --frozen --group test pytest — 245 passed.
• The issue's repro returns data: {} content: b'[{"label": "llama", "score": 0.99}]' instead of raising.

[rickstaa]

@josh please ignore this was my agent testing things.