[Store] Replace LOG(FATAL) with LOG(ERROR) for missing MC_CXL_DEV_SIZE

[stmatengss]

Problem

Two locations in real_client.cpp call LOG(FATAL) when MC_CXL_DEV_SIZE is not set:

} else {
    LOG(FATAL) << "MC_CXL_DEV_SIZE not set";  // aborts process
    return tl::unexpected(...);               // unreachable
}

A missing env var is a user configuration error and should not crash the process.

Fix

Replace both LOG(FATAL) calls with LOG(ERROR). The return tl::unexpected(ErrorCode::INVALID_PARAMS) that follows then executes correctly as a graceful error return.

Impact

• Missing MC_CXL_DEV_SIZE now returns INVALID_PARAMS instead of crashing
• No change to behavior when env var is set correctly
• No change to RAM/TCP/RDMA paths