Generate cuopt_routing.proto and VRP mapper via field_registry codegen

Summary

Bring VRP under the gRPC field_registry codegen so cpp/src/grpc/cuopt_routing.proto and the proto ↔ cpu_routing_problem_t conversion are generated (like the math-opt LP/MIP protos), replacing the hand-authored proto and the ~377-line hand-written cpp/src/grpc/grpc_routing_problem_mapper.cpp.

Follow-up to #1597, where this was intentionally deferred to keep that PR focused/landable (see thread).

Scope

• Model the routing messages in field_registry.yaml: RoutingProblem, RoutingSolverSettings, RoutingSolution, and sub-messages (CostMatrix, CapacityDimension, VehicleBreak/PerVehicleBreaks/UniformBreakDimension, MatchEntry, PrecedenceEntry, ServiceTimeEntry, RoutingObjectiveFunction, InitialSolution).
• Generate cuopt_routing.proto from the registry and delete the hand-authored file.
• Generate the proto ↔ cpu_routing_problem_t conversion and delete grpc_routing_problem_mapper.cpp.
• Extend ci/verify_grpc_codegen.sh coverage to the routing proto/mapper.

Known blockers / considerations

• The routing sub-messages fit the existing repeated_messages mechanism (cf. quadratic_constraints in field_registry.yaml).
• No proto3 map<> support in generate_conversions.py yet — RoutingSolution.objective_values is map<int32, double>. Either add map support to the generator or model it as parallel repeated key/value arrays.
• The current mapper has bespoke logic that doesn't fully fit the array-id chunked model: cost-matrix grouping by vehicle_type, capacity dimensions (name + demand + capacity), per-vehicle service times (vehicle_id = -1 default), PDP pickup/delivery indices, breaks, vehicle/order matching, precedence, and initial solutions.

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