fix: 🐛 修复 OpenAI 格式连续 tool 消息转 Anthropic 时未合并导致请求报错 (#219)

当用户以 OpenAI 格式请求 Claude 模型时，末尾连续多条 role:tool 消息会被各自
转换为独立的 user 消息，违反 Anthropic API 角色交替要求，导致 Improperly
formed request 错误。在 convertMessages 中增加连续同角色消息合并逻辑，并提取
contentToBlocks 复用简化 convertToolMessage 中的内容转换代码。

Author: NibbinNone

internal/transformer/outbound/authropic/messages.go

@@ -391,12 +391,36 @@ func convertMessages(req *model.InternalLLMRequest) []anthropicModel.MessagePara
 		}
 
 		converted := convertSingleMessage(msg, req.Messages, processedIndexes)
-		messages = append(messages, converted...)
+		for _, convertedMsg := range converted {
+			// Anthropic API 要求消息角色必须交替出现（user/assistant/user/assistant）。
+			// 当 OpenAI 格式的多个连续 tool 消息被各自转换为独立的 user 消息时，
+			// 会产生连续的同角色消息，需要合并以避免 "Improperly formed request" 错误。
+			if n := len(messages); n > 0 && messages[n-1].Role == convertedMsg.Role {
+				last := &messages[n-1]
+				last.Content = anthropicModel.MessageContent{
+					MultipleContent: append(contentToBlocks(last.Content), contentToBlocks(convertedMsg.Content)...),
+				}
+			} else {
+				messages = append(messages, convertedMsg)
+			}
+		}
 	}
 
 	return messages
 }
 
+// contentToBlocks 将 MessageContent 统一展开为 MessageContentBlock 切片。
+func contentToBlocks(c anthropicModel.MessageContent) []anthropicModel.MessageContentBlock {
+	if len(c.MultipleContent) > 0 {
+		// 返回副本，避免后续 append 污染原 slice
+		return append([]anthropicModel.MessageContentBlock(nil), c.MultipleContent...)
+	}
+	if c.Content != nil && *c.Content != "" {
+		return []anthropicModel.MessageContentBlock{{Type: "text", Text: c.Content}}
+	}
+	return nil
+}
+
 func convertSingleMessage(msg model.Message, allMessages []model.Message, processedIndexes map[int]bool) []anthropicModel.MessageParam {
 	switch msg.Role {
 	case "tool":
@@ -449,14 +473,7 @@ func convertToolMessage(msg model.Message, allMessages []model.Message, processe
 	// original Anthropic request may also include additional user content alongside tool_result.
 	if userMsg := findUserMessageByIndex(allMessages, *msg.MessageIndex); userMsg != nil {
 		userContent := buildMessageContent(*userMsg)
-		if len(userContent.MultipleContent) > 0 {
-			contentBlocks = append(contentBlocks, userContent.MultipleContent...)
-		} else if userContent.Content != nil && *userContent.Content != "" {
-			contentBlocks = append(contentBlocks, anthropicModel.MessageContentBlock{
-				Type: "text",
-				Text: userContent.Content,
-			})
-		}
+		contentBlocks = append(contentBlocks, contentToBlocks(userContent)...)
 	}
 
 	processedIndexes[*msg.MessageIndex] = true