[202511_azd] [sonic-yang-models]: Add VNet name support to BGP_NEIGHBOR YANG model

[miatttao]

Why I did it

The BGP_NEIGHBOR YANG model only accepted a VRF name (referencing BGP_GLOBALS) as the first key component. This prevented creating BGP neighbor configurations scoped to a VNet, such as BGP_NEIGHBOR|Vnet1|10.0.0.0, which is needed for overlay BGP sessions over VxLAN tunnels.

Work item tracking

• Microsoft ADO (number only):

How I did it

Changed the vrf_name leaf type in BGP_NEIGHBOR_LIST and BGP_NEIGHBOR_AF_LIST from a plain leafref to a union of two leafref types — one pointing to BGP_GLOBALS_LIST (existing VRF path) and one pointing to VNET_LIST (new VNet path). This follows the identical pattern already used in sonic-bgp-peerrange for the same requirement.

Files changed:

• yang-models/sonic-bgp-neighbor.yang — import sonic-vnet, change vrf_name to union
  leafref in both BGP_NEIGHBOR_LIST and BGP_NEIGHBOR_AF_LIST
• doc/Configuration.md — document VNet key format and add example
• tests/files/sample_config_db.json — add Vnet55|10.0.0.0 BGP_NEIGHBOR and
  BGP_NEIGHBOR_AF entries
• tests/yang_model_tests/tests/bgp.json — add BGP_NEIGHBOR_VNET_VALID and
  BGP_NEIGHBOR_NEG_VNET_NOT_EXIST test descriptors
• tests/yang_model_tests/tests_config/bgp.json — add full test data for both new cases

How to verify it

Run the existing YANG model tests:

cd src/sonic-yang-models
python3 -m pytest tests/yang_model_tests/test_yang_model.py -v -k "BGP_NEIGHBOR"

Which release branch to backport (provide reason below if selected)

• 202305
• 202311
• 202405
• 202411
• 202505
• 202511
• 202512
• 202605
• 202608
• 202506

Tracking issue/work item for backport/cherry-pick request (GitHub issue or Microsoft ADO):
Failure type:

Tested branch

• master
• 202305
• 202311
• 202405
• 202411
• 202505
• 202511
• 202512
• 202605
• 202608

Test result

Description for the changelog

Link to config_db schema for YANG module changes

A picture of a cute animal (not mandatory but encouraged)

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