fix(anthropic): improve MCP tool message handling and error resilience

修复Anthropic协议中MCP工具消息处理问题，增强错误恢复能力：
- 改进MessageContent的JSON解析，支持原始JSON数据保留
- 移除对空内容的过滤，避免丢失MCP工具消息
- 增加详细调试日志，便于问题排查
- 优化错误响应处理，保留响应体信息
- 修复路由转发时可能的数据丢失问题

fix(anthropic): improve MCP tool message handling and error resilience
- Enhance MessageContent JSON parsing to preserve raw JSON data
- Remove empty content filtering to prevent MCP tool message loss
- Add detailed debug logging for troubleshooting
- Optimize error response handling with response body preservation
- Fix potential data loss during routing

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

relay/adaptor/anthropic/model.go

@@ -36,6 +36,12 @@ type MessageContent struct {
 
 // UnmarshalJSON implements json.Unmarshaler to handle both string and array formats
 func (m *MessageContent) UnmarshalJSON(data []byte) error {
+	// Skip empty data or null
+	if len(data) == 0 || string(data) == "null" {
+		m.value = ""
+		return nil
+	}
+
 	// Try to unmarshal as string first
 	var str string
 	if err := json.Unmarshal(data, &str); err == nil {
@@ -50,6 +56,14 @@ func (m *MessageContent) UnmarshalJSON(data []byte) error {
 		return nil
 	}
 
+	// For routing purposes, store raw JSON if we can't parse it
+	// This ensures we don't lose any data during forwarding
+	var raw json.RawMessage
+	if err := json.Unmarshal(data, &raw); err == nil {
+		m.value = raw
+		return nil
+	}
+
 	return fmt.Errorf("message content must be either a string or an array of content blocks")
 }
 
@@ -73,6 +87,14 @@ func (m MessageContent) ToContentArray() []Content {
 		}}
 	case []Content:
 		return v
+	case json.RawMessage:
+		// Try to parse raw JSON as Content array
+		var arr []Content
+		if err := json.Unmarshal(v, &arr); err == nil {
+			return arr
+		}
+		// If that fails, return empty array to avoid breaking the routing
+		return []Content{}
 	default:
 		return []Content{}
 	}