fix(embedding): resolve race condition vulnerabilities in dimension detection

- Add Promise-based dimension detection caching in both OpenAI and Ollama providers
- Prevent concurrent embed/embedBatch calls from triggering multiple API requests
- Implement race condition-safe ensureDimensionDetected() methods
- Fix state management in setModel() to properly reset all detection flags
- Maintain backward compatibility with existing API interfaces
- Addresses performance and reliability issues identified in zen audit

Author: MapleEve