fix(openai): preserve structured and stringified custom tool outputs during Responses conversion

- Add `setCustomToolCallOutputContent` for `custom_tool_call_output` handling in the request translator.
- Parse JSON-encoded `output` strings and route image-based tool-output parts through existing image/function-call conversion path.
- Keep non-image outputs as text via `responsesToolOutputText`, including plain text and unrecognized/invalid image payloads.

Closes: #4902