T7775: Add fix AES-GCM-256 misidentification

[sever-sever]

Change Summary

VPP was incorrectly identifying AES-GCM-256 IPsec SAs as AES-GCM-128,
causing a cryptographic key material mismatch with peer implementations
like StrongSwan. This resulted in encryption/decryption failures and
non-functional IPsec tunnels.

Root cause: The libnl xfrmnl_sa_get_aead_params() function reports only
the cipher key length (e.g., 128 bits for what should be AES-GCM-256)
in the aead_key_len parameter, while extracting the full key material
including padding into the key buffer (46 bytes instead of the expected 36).

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Other (please describe):

Related Task(s)

• https://vyos.dev/T7775

Related PR(s)

Proposed changes

How to test

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly