fix: use unixgram (DGRAM) instead of unix (STREAM) for socket output

- Change net.DialTimeout from "unix" to "unixgram"
- Fixes "protocol wrong type for socket" error
- DGRAM sockets are connectionless, better suited for log shipping
- Update test to use net.ListenUnixgram instead of net.Listen

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

internal/output/writers.go

@@ -259,6 +259,7 @@ type UnixSocketWriter struct {
 	errorCallback     ErrorCallback
 	consecutiveFailures int
 	failuresMu        sync.Mutex
+	networkType       string // "unix" for STREAM, "unixgram" for DGRAM
 }
 
 // NewUnixSocketWriter creates a new UNIX socket writer with reconnection logic
@@ -300,7 +301,8 @@ func NewUnixSocketWriterWithConfig(socketPath string, dialTimeout, writeTimeout
 	go w.processQueue()
 
 	// Try initial connection silently (socket may not exist yet - that's okay)
-	conn, err := net.DialTimeout("unix", socketPath, w.dialTimeout)
+	// Use unixgram (DGRAM) for connectionless UDP-like socket communication
+	conn, err := net.DialTimeout("unixgram", socketPath, w.dialTimeout)
 	if err == nil {
 		w.conn = conn
 	}
@@ -399,7 +401,8 @@ func (w *UnixSocketWriter) writeWithReconnect(data []byte) error {
 		if w.conn != nil {
 			return nil
 		}
-		conn, err := net.DialTimeout("unix", w.socketPath, w.dialTimeout)
+		// Use unixgram (DGRAM) for connectionless UDP-like socket communication
+		conn, err := net.DialTimeout("unixgram", w.socketPath, w.dialTimeout)
 		if err != nil {
 			return fmt.Errorf("failed to connect to socket %s: %w", w.socketPath, err)
 		}