fix(call_runner): stop asking apps for JSON

[rickstaa]

Stacked on #51 (base rs/call-runner-raw-bytes), which handles the response side. Without this, its non-JSON branch is unreachable for the apps it targets.

_json_request_parts hardcodes Accept: application/json on every SDK request. Right for the control plane, but call_runner talks to arbitrary app code, so it asserts a preference the caller never made, and an upstream that content-negotiates obliges. From the api-proxy example, which advertises raw JPEG bytes:

no Accept header          -> image/jpeg,       28819 bytes
Accept: application/json  -> application/json, base64 PNG in a JSON string

The client wrote 1.9 MB of base64 text to a .jpg.

App calls now send Accept: */* and let the app, or the upstream it fronts, pick. Control plane untouched, http.py unchanged. */* rather than omitting the header: aiohttp sends it anyway, so this only makes the choice visible.

JSON apps are unaffected, since json_response sets its own content type and the buffered path branches on the response. No headers argument for now: no caller needs to negotiate, and it can be added later without breaking anyone.

Verified against api-proxy: a real Hugging Face call now returns a 1024x1024 image/jpeg. Suite passes (16).

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[j0sh]

Thanks, I'm trying to remember if this was put in place for a reason. Will take a closer look