Make changes to avoid duplicate AllowedSnssai

[gab-arrobo]

No description provided.

[Copilot]

Pull request overview

This PR avoids duplicate Allowed NSSAI entries when deriving NGAP/NAS AllowedNssai data, and adds regression tests for the deduplication behavior.

Changes:

• Adds a shared NGAP AllowedNSSAI builder that deduplicates S-NSSAIs and enforces the NGAP maximum of 8 items.
• Updates Initial Context Setup, Handover Request, and Path Switch Request Acknowledge to use UE-specific allowed NSSAI data.
• Adds GMM logic/tests to rebuild requested NSSAI without retaining duplicates.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
ngap/message/build.go	Adds deduplicating AllowedNSSAI construction and uses it in NGAP message builders.
ngap/message/build_test.go	Adds tests for NGAP AllowedNSSAI deduplication, max size enforcement, and UE-specific NSSAI usage.
gmm/handler.go	Refactors requested NSSAI handling into a helper that rebuilds allowed NSSAI without duplicates.
gmm/requested_nssai_test.go	Adds a regression test for requested NSSAI deduplication.

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