[langchain] fix OpenAIAssistantRunnable.create_assistant (#19081)
- **Description:** OpenAI assistants support some pre-built tools (e.g.,
`"retrieval"` and `"code_interpreter"`) and expect these as `{"type":
"code_interpreter"}`. This may have been upset by
https://github.com/langchain-ai/langchain/pull/18935
- **Issue:** https://github.com/langchain-ai/langchain/issues/19057