[bwachman]

Summary

• On Codex, a response with several tool calls now delivers every call's arguments even when the backend sends them only on response.output_item.done and streams no function_call_arguments.delta events. Calls that do stream deltas are unchanged.
• The replay lives next to withParallelToolCalls, the host override that turns parallel calls on for Codex, because that override is what exposes this backend behavior; the packaged parser reads arguments from deltas alone.

Without this, the Codex backend's no-delta path reaches the tools with every call's arguments as {} (the harness assembles an empty buffer as {}), so parallel fan-outs such as several spawn_agent calls fail argument validation and nothing starts.

Verification

• bun run check passes (lint, typecheck, dead exports, build, full test suite: 8665 pass, 0 fail)
• New golden fixture parallel-tools-arguments-on-done.json reproduces the no-delta shape; its test fails without the change and passes with it, and the existing streamed-call fixture produces the same events as before
• Replaying a captured live Codex response from the no-delta path yields all three call arguments with the change and none without it
• A live headless run on codex/plus (gpt-5.6-terra) that hit the no-delta path read three files in parallel with full arguments on the first attempt

Fixes CL-10582

[corbits-triage]

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[linear-code]

CL-10582