fix: set RatType correctly during registration

[donivtech]

Problem

During a normal UE registration, ue.RatType is never set. The only code path that assigns it is CreateUEContextProcedure in producer/ue_context.go (inter-AMF relocation), and it does:

go ue.RatType = ueContextCreateData.UeContext.RestrictedRatList[0] // minItem = -1

That is wrong on two counts:

• RestrictedRatList contains RAT types the UE is not allowed to use — it's the opposite of what RatType means.
• The list's minimum item count is -1 (i.e. it can be empty or nil), so indexing [0] can panic.

As a result, several SBI consumers (Nsmf_PDUSession CreateSMContext, Namf_Communication UE context management, location reporting, MT notifications) send an empty ratType field.

Fix

• Set ue.RatType in HandleRegistrationRequest based on access type:
  • 3GPP access + NrLocation → NR
  • 3GPP access + EutraLocation → EUTRA
  • Non-3GPP access → WLAN
• Remove the buggy RestrictedRatList[0] assignment in CreateUEContextProcedure .

Test plan

• go build ./... clean
• go vet ./... clean
• go test ./gmm/... ./producer/... pass
• make lint — 0 issues

[donivtech]

Holding this until PR #684 (NR NTN access detection) gets maintainer feedback — naming/architecture decisions there can also apply here. Branch kept on the fork; will reopen when #684 lands.