phsl / new-api

refactor: Update SystemInstructions type in GeminiChatRequest and adjust handling in CovertGemini2OpenAI

- Changed SystemInstructions from *GeminiPart to *GeminiChatContent in GeminiChatRequest for improved structure.
- Updated CovertGemini2OpenAI function to accommodate the new SystemInstructions type, ensuring proper handling of message content.