Add Type Validation for API Key and Auth Token to Prevent Common Configuration Errors

[MikeSoft]

While integrating the Anthropic API client, I encountered an issue where I mistakenly passed the api_key as a list instead of a string. This led to several hours of debugging to identify the root cause of the issue, as the current implementation does not explicitly validate the type of api_key and auth_token upon instantiation of the Anthropic client class.

To prevent such issues for other developers and to enhance the usability of the API client, I propose adding explicit type validation for both api_key and auth_token parameters within the init method of the Anthropic class. This small yet impactful change ensures that developers are immediately alerted with a clear and informative error message if the types of these parameters do not match the expected string type.

The proposed change has been implemented as follows:

[rattrayalex]

Thank you for sharing your experience and putting up a PR! We'll take a look.