feat(core): improve code quality based on GitHub Copilot suggestions

- Extract magic number 768 to DEFAULT_OLLAMA_DIMENSION constant
- Extract 'api.openai.com' to OPENAI_API_DOMAIN constant
- Refactor long error messages to static methods for better maintainability
- Update test expectations to match new error message structure
- Improve code readability and reduce duplication

Author: MapleEve