[somaz94]

Loading a config through the v2jsonpb loader panics with a nil pointer dereference as soon as it meets a field name the message does not define. When the config comes from stdin, v2ray tries every loader in turn and v2jsonpb panics before the others get a chance, so echo '{ "status": {} }' | v2ray test crashes instead of falling through to the next format. This is what #3598 reports.

ByJSONName and ByTextName on V2JsonProtobufFollowerFields wrap the inner lookup in V2JsonProtobufFollowerFieldDescriptor even when it finds nothing, so protojson gets a non-nil descriptor holding nil. protojson only reports an unknown field, and only retries with the proto field name, when the lookup returns nil, so it calls fd.Number() on the empty wrapper instead. Snake case keys such as receiver_settings hit the same panic for the same reason.

Both methods now return nil when the inner lookup finds nothing. The new TestLoadFieldNames covers a JSON name, a proto name, and an unknown field at the top level, in a list element and inside an Any value.

Validation: the new test panics without the change and passes with it, on go1.27.1 and go1.26.1. The stdin command above panicked before and prints Configuration OK. after. golangci-lint with .github/linters/.golangci.yml reports 0 issues, and go mod tidy leaves go.mod and go.sum unchanged. go test ./... passes except TestServiceSubscribeRoutingStats in app/router/command, which failed once in the full run, passes 10 out of 10 runs on its own with and without this change, and does not depend on v2jsonpb.

Fixes #3598