Add support for API key indexing in OpenAI compatibility clients
- Updated `NewOpenAICompatibilityClient` to accept `apiKeyIndex` for managing multiple API keys.
- Modified client instantiation loops to initialize one client per API key.
- Adjusted client ID format to include `apiKeyIndex` for unique identification.
- Removed API key rotation logic within `GetCurrentAPIKey`.
- Updated `.gitignore` to include `AGENTS.md`.