fix: use authoritative final arguments for OpenAI Responses tool calls

[toffee-desuwa]

Related Issue

Resolve #1455

Problem

See linked issue. Both Responses adapters' yieldFinalArgumentsSuffix() only accept the final tool-call arguments when they equal, or are a byte-prefix of, the accumulated streamed delta. A Responses-compatible provider that returns corrected full JSON in function_call_arguments.done / output_item.done that is not a prefix of the streamed partial makes the adapter throw and aborts the whole agent turn.

What changed

Both Responses adapters (packages/kosong/src/providers/openai-responses.ts and packages/agent-core-v2/src/app/llmProtocol/providers/openai-responses.ts) now buffer tool-call arguments per stream index instead of committing the tool call from the streamed deltas. Authority order: output_item.done.item.arguments > function_call_arguments.done.arguments > accumulated delta (used only when both are absent). A single complete, indexed tool_call_part is emitted at item/response completion.

Behavior change: tool-call arguments are now emitted once at output_item.done / end of stream rather than streamed per delta. The consumers are append-only (target.arguments + argumentsPart, with no primitive to replace already-appended text), so a corrective non-prefix final cannot be un-applied downstream — buffering is the only correct fix, and it matches the authority order the linked issue recommends. A guard also rejects starting a new function_call header while an unindexed call is still unresolved; it is only reachable for non-conforming providers that omit both item_id and output_index, and it turns a previously silent argument clobber into a fail-loud error.

Tests (Vitest) cover: final arguments overriding the streamed delta, output_item.done precedence, done-absent fallback to the accumulated delta, and parallel tool-call indices not cross-streaming.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update.

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