Do not ignore SO_ERROR value on connect

Otherwise we will mark the connection as successful and a subsequent read/write will return EOF. In reality, the connect
syscall failed. Checking the return value of Getsockoptint(... SO_ERROR) will tell us whether the connect syscall
succeeded. A return value of 0 means success - anything else can be interpreted by syscall.Errno and means failure.

I was mislead into thinking the returned `err` from `Getsockoptint` is the actual socket error - in reality, it's whether the `getsockopt` syscall succeeded or not.

- Golang src: https://github.com/golang/go/blob/04879acdebbb08bdca00356f043d769c4b4375ce/src/syscall/syscall_unix.go#L312
- Sanity check that uninit values are set to 0: https://go.dev/play/p/vCLjDd6WwL-
- syscall.Errno is safe for any input value: https://cs.opensource.google/go/go/+/refs/tags/go1.23.3:src/syscall/syscall_unix.go;l=110

internal/socket_unix.go

@@ -151,10 +151,13 @@ func connect(fd int, remoteAddr net.Addr, timeout time.Duration, opts ...sonicop
 			return sonicerrors.ErrTimeout
 		}
 
-		_, err = syscall.GetsockoptInt(fd, syscall.SOL_SOCKET, syscall.SO_ERROR)
+		socketErr, err := syscall.GetsockoptInt(fd, syscall.SOL_SOCKET, syscall.SO_ERROR)
 		if err != nil {
 			return os.NewSyscallError("getsockopt", err)
 		}
+		if socketErr != 0 {
+			return syscall.Errno(socketErr)
+		}
 	}
 
 	return nil