phsl / new-api

- Introduced `GEMINI_VISION_MAX_IMAGE_NUM` to README files for better user guidance.
- Updated `env.go` to retrieve the maximum image number from environment variables, defaulting to 16.
- Modified image handling logic in `relay-gemini.go` to respect the new configuration, allowing disabling of the limit by setting it to -1.
- Removed hardcoded constant for maximum image number in `constant.go` to streamline configuration management.