fix: fullURL endpoint generation (#817)

chat.go

@@ -358,7 +358,12 @@ func (c *Client) CreateChatCompletion(
 		return
 	}
 
-	req, err := c.newRequest(ctx, http.MethodPost, c.fullURL(urlSuffix, request.Model), withBody(request))
+	req, err := c.newRequest(
+		ctx,
+		http.MethodPost,
+		c.fullURL(urlSuffix, withModel(request.Model)),
+		withBody(request),
+	)
 	if err != nil {
 		return
 	}