fix(notifier): add OAuth2 token to namf-callback outbound calls; fix SCTP EBADF teardown

[solar224]

Problem

• AMF sent namf-callback notifications (N1/N2 transfer failure, N1 message notify, UE context transfer) using context.Background(), bypassing OAuth2. Receiving NFs rejected the requests with 401.
• SCTP teardown incorrectly called HandleConnectionError twice when the file descriptor was already closed (EBADF), producing spurious error logs.

Changes

• notifier/n1n2message.go, notifier/subscription.go, notifier/ue_context.go: replace context.Background() with GetTokenCtx("namf-callback", <target NF type>) for all outbound callback calls
• server.go: register namf-callback route group
• pkg/factory/config.go: add namf-callback to ServiceNameList validator
• pkg/service/init.go: service init adjustments
• ngap/service/service.go: add case syscall.EBADF → debug log + return, preventing double error handling