Add test_parallel_tool_calls_comprehensive_streaming_integration which
synthesizes the full 10-event Responses API SSE sequence with split
argument deltas and asserts all fix invariants together:
1. output_item.done emits no finish_reason (no premature stream end)
2. Each call_id appears exactly once (no duplicate tool_call chunks)
3. Split argument deltas assemble to correct final JSON
4. Exactly one finish event, at the terminal response.completed chunk
5. Parallel tool calls have distinct indices (output_index 0 and 1)
All 24 unit tests pass.