gpt-5.4 returns `arguments` as a JSON object for some tools (e.g.
apply_patch, tool_search), but ResponsesOutput.Arguments was typed as
string, causing json.Unmarshal to fail on every such SSE event.

The failure silently dropped all output_item events and the
response.completed event from the forwarded stream, so:
- the client never received tool call content
- usage/token counts could not be extracted (billed as 0)
- client reported "stream closed before response.completed"

Fix: change Arguments to json.RawMessage and add GetArguments() which
normalises both forms to a plain string (unescapes JSON strings,
returns raw bytes for objects).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>