Forward libcanard TX deadlines through the CAN driver vtable

[Copilot]

cy_can_vtable_t::tx_classic and tx_fd were dropping the libcanard transmission deadline even though the CAN transport already receives it. This updates the public CAN driver API so platform backends can observe the deadline directly, matching the FluxGrip-facing integration model.

• Public CAN vtable API

  • Extend tx_classic and tx_fd to accept a canard_us_t deadline.
  • Update the callback contract in cy_can.h to make deadline-aware transmission explicit.

• Deadline propagation

  • Pass the libcanard deadline through cy_can.c instead of discarding it before dispatching to the backend vtable.

• Backend and test support

  • Update the SocketCAN backend to the new callback signature.
  • Update CAN test doubles to capture the last deadline seen on classic and FD TX paths.
  • Add focused assertions covering both classic and FD publication paths.

• Example

  typedef struct
  {
      bool (*tx_classic)(void* user,
                         canard_us_t deadline,
                         uint_least8_t iface_index,
                         uint32_t can_id,
                         const void* data,
                         uint_least8_t len);
  
      bool (*tx_fd)(void* user,
                    canard_us_t deadline,
                    uint_least8_t iface_index,
                    uint32_t can_id,
                    const void* data,
                    uint_least8_t len);
  } cy_can_vtable_t;