refactor: linter + formatter

- Updated the constructor to retrieve the API key from environment variables if not provided.
- Added a validation step to ensure the API key is present, raising a ValueError if missing.
- Consolidated header management by using the instance's headers attribute for API requests, improving code clarity and maintainability.

- Removed the async_client attribute and its related initialization logic.
- Consolidated API key and headers setup into the constructor for clarity.
- Enhanced code readability by eliminating redundant code and improving structure.

- Replaced async_client with aiohttp.ClientSession for making asynchronous HTTP requests.
- Added headers for API requests to enhance security and compatibility.
- Simplified error handling by removing checks for async_client initialization.
- Improved code readability and maintainability by consolidating request logic.