Refactor LangSmith API key handling and connection check

- Changed `langSmithApiKey` to `langSmithApiKeyId` in PluginData interface and data store.
- Updated the way the API key is stored and retrieved using managed secrets.
- Added a new method to check the LangSmith API connection with user feedback.
- Introduced a `SecretSelect` component for selecting the API key from Obsidian's keychain.
- Enhanced the settings UI to include a button for checking the API connection status.
- Implemented loading states and error handling for the connection check process.