Fix error handler with using unsubscribed response body in exception message

[jitokim]

Fix error handler with using unsubscribed response body in exception message in AnthropicApi class.

When an error occurs while calling the chatCompletionStream function, It's difficult to know the cause.

AS-IS
RuntimeException's message is a toString of an object (Mono<String>).

Response exception, Status: [401 UNAUTHORIZED], Body:[checkpoint("Body from POST https://api.anthropic.com/v1/messages [DefaultClientResponse]")]

TO-BE
Replace the RuntimeException's message with the response body.

Response exception, Status: [401 UNAUTHORIZED], Body:[{"type":"error","error":{"type":"authentication_error","message":"invalid x-api-key"}}]

[markpollack]

Thanks! merged in fb2e752