fix(ngap): route remaining FetchRanUeContext lookups through nil-safe helper

[donivtech]

Description

#666 introduced the nil-safe helper findRanUeByRanNgapID() — it returns nil when the RANUENGAPID IE pointer is nil instead of dereferencing .Value — and converted most procedure-code cases in FetchRanUeContext to use it. Three cases were missed and still call ran.RanUeFindByRanUeNgapID(rANUENGAPID.Value) directly:

• HandoverNotification
• HandoverPreparation
• PDUSessionResourceModify (Response)

In each, the in-loop check only catches a RANUENGAPID IE that is present with a nil value. If the IE is entirely absent from the message, rANUENGAPID stays nil and the subsequent .Value dereference panics the AMF — the same malformed-input crash class #666 set out to close.

This routes those three lookups through findRanUeByRanNgapID() like the rest of the function, making FetchRanUeContext uniformly nil-safe.

Testing

• go build ./..., go vet ./..., go test ./... all pass
• golangci-lint run --new-from-rev=main → 0 issues.
• gofmt clean

[Copilot]

Pull request overview

This pull request completes the migration in FetchRanUeContext to use the nil-safe findRanUeByRanNgapID() helper for RAN UE context lookup, preventing panics when RANUENGAPID is absent from malformed NGAP messages.

Changes:

• Updated HandoverNotification to use findRanUeByRanNgapID() instead of dereferencing rANUENGAPID.Value.
• Updated HandoverPreparation to use findRanUeByRanNgapID() instead of dereferencing rANUENGAPID.Value.
• Updated PDUSessionResourceModify (SuccessfulOutcome / response path) to use findRanUeByRanNgapID() instead of dereferencing rANUENGAPID.Value.

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[gab-arrobo]

Should we make this change across the codebase? There are other 20+ occurrences of this

-			ranUe = ran.RanUeFindByRanUeNgapID(rANUENGAPID.Value)
+			ranUe = findRanUeByRanNgapID(ran, rANUENGAPID)

[donivtech]

Agreed. It's worth doing across the codebase: beyond the 4 handlers in #731, there are 6 more that still panic on an absent ID IE (UplinkNasTransport, PDUSessionResourceReleaseResponse, HandoverCancel, NasNonDeliveryIndication, UERadioCapabilityInfoIndication, UEContextReleaseComplete). I'll do a sweep converting all the raw RanUeFindBy{Ran,AmfUe}NgapID(...Value) calls to the two helpers.

[donivtech]

Implemented in #731. Expanded from the original 4 handlers to all 10 that actually panic on a missing NGAP-ID IE. Full breakdown's in the PR description.