perf(translator): batch assemble translation arrays before writing JSON

- Refactor Gemini/OpenAI/Codex/Claude/Antigravity interaction and response translators to collect `parts`, `content`, `steps`, `tool_calls`, and `output` items in slices, then set each array once via `translatorcommon` helpers.
- Replace repeated incremental `sjson.SetRawBytes(..., ".-1", ...)` mutations with single-shot raw-array updates to avoid repeated rewrites.
- Keep existing step/content semantics while reducing ordering/overwrite risks when merging multi-fragment data (including thought/signature and web-search/function-call blocks).