Use uint64_t instead of size_t in wire protocol struct (#40412)

LX_INIT_START_SOCKET_RELAY::BufferSize used size_t, which varies by platform (4 bytes on 32-bit, 8 on 64-bit). All other wire protocol structs in lxinitshared.h use fixed-width integer types. While both Windows and Linux sides are currently 64-bit (so size_t is 8 bytes on both), using size_t in a serialized cross-boundary struct violates wire protocol conventions and would break if either side ever runs in a 32-bit context. Replace with uint64_t for consistency with the rest of the wire protocol and future-proofing. Co-authored-by: Ben Hillis <benhill@ntdev.microsoft.com> Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>