fix(translator): emit function_call_arguments.done and make function call stream events idempotent

- Ensure `response.function_call_arguments.done` is emitted with final arguments (including `{}` for empty args).
- Track per-call state so duplicate `step.start` / `step.stop` / `arguments_delta` events do not re-emit redundant output.
- Populate `response.output_item.done` and completion payloads from a single normalized function-call argument source.

Closes: #4742