fix(nebula_core_hw_interfaces): improve CAN FD frame validation and type safety

Signed-off-by: David Wong <david.wong@tier4.jp>

Author: drwnz

src/nebula_core/nebula_core_hw_interfaces/include/nebula_core_hw_interfaces/nebula_hw_interfaces_common/connections/can.hpp

@@ -297,7 +297,7 @@ class CanSocket
    * @return Number of bytes received.
    * @throws SocketError if recvmsg fails.
    */
-  ssize_t receive_frame_with_metadata(void * frame_ptr, size_t frame_size, RxMetadata & metadata)
+  size_t receive_frame_with_metadata(void * frame_ptr, size_t frame_size, RxMetadata & metadata)
   {
     struct iovec iov;
     struct msghdr msg;
@@ -315,15 +315,15 @@ class CanSocket
     msg.msg_controllen = sizeof(ctrl);
     msg.msg_flags = 0;
 
-    ssize_t recv_result;
+    ssize_t recv_result{};
     do {
       recv_result = recvmsg(sock_fd_.get(), &msg, 0);
     } while (recv_result == -1 && errno == EINTR);
 
     if (recv_result < 0) throw SocketError(errno);
 
     metadata.timestamp_ns = extract_timestamp(msg);
-    return recv_result;
+    return static_cast<size_t>(recv_result);
   }
 
   /**