VRP: transparent remote execution via CUOPT_REMOTE_HOST/PORT (solve parity with LP/MIP)

Follow-up to #1597. LP/MIP support transparent remote execution: is_remote_execution_enabled() (checks CUOPT_REMOTE_HOST/CUOPT_REMOTE_PORT, see pdlp/backend_selection.cpp) makes the normal solve_lp()/solve_mip() route to solve_lp_remote()/solve_mip_remote() — non-invasive, works from any interface (C/Python/CLI) without code changes.

VRP has no equivalent: routing's Solve() never checks the env or calls a remote backend. #1597 adds only the explicit cuopt.grpc.routing.RoutingClient (you call it directly). Note CUOPT_GRPC_SERVER (used by the RoutingClient tests) is a different knob from the transparent CUOPT_REMOTE_HOST/CUOPT_REMOTE_PORT path.

Scope

• Add solve_vrp_remote() (host cpu_routing_problem_t → gRPC submit → RoutingSolution), mirroring solve_lp_remote/solve_mip_remote.
• Hook is_remote_execution_enabled() into routing's Solve() entry point so a GPU-less routing.Solve(...) transparently runs on a remote server when CUOPT_REMOTE_HOST/CUOPT_REMOTE_PORT are set.

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