perf(translator): batch assemble response arrays and merge multi-choice parts

- Refactor multiple OpenAI/Gemini/Codex/Claude/Antigravity/Interactions response translators to collect output elements in slices and set arrays once via `translatorcommon.JoinRawArray`.
- Fix non-stream OpenAI→Gemini part handling so fields (`reasoning`, `content`, `functionCall`) from multiple choices overlay correctly on shared parts instead of being overwritten.
- Preserve explicit empty message fields in Gemini→OpenAI chat-completion conversion (`content`/`reasoning_content`) and add tests covering empty-text and cross-choice overlay cases.