Inline response recording and remove process_items indirection

[aibrahim-oai]

• Inline response recording during streaming: run_turn now records items as they arrive instead of building a ProcessedResponseItem list and post‑processing via process_items.
• Simplify turn handling: handle_output_item_done returns the follow‑up signal + optional tool future; needs_follow_up is set only there, and in‑flight tool futures are drained once at the end (errors logged, no extra state writes).
• Flattened stream loop: removed process_items indirection and the extra output queue
• • Tests: relaxed tool_parallelism::tool_results_grouped to allow any completion order while still requiring matching call/output IDs.

[aibrahim-oai]

@codex review this

[jif-oai]

Can you make sure this fixes the issue described here? https://github.com/openai/codex/pull/7038