enforce anthropic's required alternating user/assistant roles

anthropic.go

@@ -65,9 +65,19 @@ func (a anthropic) requestToAnthropicRequest(req Request) anth.MessagesRequest {
 					Data:      msg.ImageBase64,
 				}))
 			}
+
+			// if this has the same role as the previous message, we can append it to the previous message
+			// as anthropic expects alternating assistant and user roles
+
+			if len(msgs) > 0 && msgs[len(msgs)-1].Role == role {
+				m2 := &msgs[len(msgs)-1]
+
+				m2.Content = append(m2.Content, m.Content...)
+			} else {
 				msgs = append(msgs, m)
 			}
 		}
+	}
 
 	for _, tool := range req.Toolbox {
 		res.Tools = append(res.Tools, anth.ToolDefinition{