fix: backfill tool call data in chat stream conversion

[MiaoZang]

Summary

• Backfill function name and arguments when Responses streams emit them in response.function_call_arguments.done or response.output_item.done.
• Track whether a tool call name/arguments have already been sent to avoid duplicate deltas.

Verification

• go test ./internal/pkg/apicompat -run 'TestResponsesEventToChatChunks_ToolCall' -count=1
• git diff --check

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[MiaoZang]

I have read the CLA Document and I hereby sign the CLA