[bgp] Support BGP confederation in update replication test

[bingwang-ms]

Description of PR

Summary:
Improve tests/bgp/test_bgp_update_replication.py so the test can establish ExaBGP neighbors correctly on BGP confederation topologies.

Fixes # (issue)

Type of change

• Bug fix
• Testbed and Framework(new/improvement)
• New Test case
  • Skipped for non-supported platforms
• Test case improvement

Back port request

• 202205
• 202305
• 202311
• 202405
• 202411
• 202505
• 202511

Approach

What is the motivation for this PR?

test_bgp_update_replication.py currently brings up BGP peers by using only minigraph_bgp_asn as the DUT-facing ASN. That works for regular topologies, but it does not work correctly when the DUT is running BGP confederation.

How did you do it?

• read the confederation ASN from the DUT with get_bgp_confed_asn()
• pass confed_asn into each BGPNeighbor
• enable use_vtysh for FabricSpineRouter when confederation is enabled, matching the existing confed-aware pattern already used by nearby BGP tests
• keep existing behavior unchanged for non-confederation topologies

How did you verify/test it?

• python -m flake8 --max-line-length=120 tests\\bgp\\test_bgp_update_replication.py
• python -m py_compile tests\\bgp\\test_bgp_update_replication.py
• attempted python -m pytest --collect-only tests\\bgp\\test_bgp_update_replication.py -q, but collection is blocked in this Windows environment because repo test helpers import Linux-only fcntl

The change is also validated on a physical testbed

Any platform specific information?

This change is only relevant for confederation-enabled topologies, especially FT2 FabricSpineRouter cases that require the existing vtysh configuration path.

Supported testbed topology if it's a new test case?

Not a new test case.

Documentation

No documentation update is needed because this changes existing test setup logic only.

[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[StormLiangMS]

Code Review

✅ LGTM — Correctly adds confed_asn and use_vtysh support for FabricSpineRouter when BGP confederation is enabled. Follows the existing confed-aware patterns used in nearby BGP tests. Clean, targeted change with physical testbed validation.

[mssonicbld]

Cherry-pick PR to 202511: #23150