test: assert list_services response uses proper protobuf structs

[yordis]

• Protobuf 0.16 deprecated implicit coercion of plain maps to message structs — returning %{service: [...]} instead of %Grpc.Reflection.V1.ListServiceResponse{service: [...]} from reflect/2 triggers deprecation warnings today and will raise in a future version. The implementation was corrected in bump protobuf, and partition process.ex to fix warnings #76, but no tests guarded the contract.
• The existing integration tests for list_services used %{service: service_list} (plain map pattern) and Map.get(&1, :name), both of which match structs and plain maps equally — so they could not detect a regression back to plain maps even though the roundtrip encode/decode masks the problem anyway.
• Adds a direct unit test for GrpcReflection.Server.V1.reflect/2 that bypasses the gRPC roundtrip and asserts %Grpc.Reflection.V1.ListServiceResponse{} and %Grpc.Reflection.V1.ServiceResponse{} struct types explicitly.
• Updates both v1 and v1alpha integration tests to use struct patterns and field access instead of plain map patterns and Map.get/2.

[yordis]

@mjheilmann just adding a test to add a bit of more guardrails