[uma/lma] Add BGP confederation route-propagation tests

[wenyiz2021]

This pull request adds a comprehensive set of BGP confederation route-propagation tests for lma (LowerMgmtAggregator) and uma (UpperMgmtAggregator) devices. The tests verify correct AS presentation, AS-path integrity, and cross-boundary route propagation within a BGP confederation, ensuring the devices behave according to BGP confederation standards. The tests are designed to be safe to run from any control host and do not require neighbor VM access.

Test Coverage for Key Confederation Behaviors

• Implemented tests to verify that the DUT presents its member AS to confederation-internal peers and the confederation identifier to external peers (test_confed_local_as_presentation).
• Added checks to ensure member sub-AS numbers only appear inside AS_CONFED_SEQUENCE segments and that the confederation identifier does not leak into the local RIB (test_confed_as_path_integrity).
• Verified that the default route originated on one side of the confederation boundary is learned and re-advertised

LMA DUT (-10) — member 64589, confed id 64582, confed peers {64588}:
AS presentation: external peers (AS 65300, M2/M3) see localAs=64582; confed-internal peers (AS 64588, UMA) see localAs=64589.
Cross-boundary propagation: default learned from confed-internal UMA → re-advertised to external M2/M3.
IPv4: origin 10.0.0.1, 10.0.0.3 → advertised to 10.0.0.137/139/141/143
IPv6: origin fc00::2, fc00::6 → advertised to fc00::112/116/11a/11e

UMA DUT (-9) — member 64588, confed id 64582, confed peers {64589}:
AS presentation: external peers (RWA AS 65200, M1 AS 65300) see localAs=64582; confed-internal peers (AS 64589, LMA) see localAs=64588.
Cross-boundary propagation (mirror direction): default learned from external RWA → re-advertised to confed-internal LMA.
IPv4: origin 10.0.0.1 → advertised to 10.0.0.137/139/141/143
IPv6: origin fc00::2 → advertised to fc00::112/116/11a/11e

Description of PR

Summary:
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

• 202311
• 202405
• 202411
• 202505
• 202511
• 202512
• 202605

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

Tested branch

• master
• 202311
• 202405
• 202411
• 202505
• 202511
• 202512
• 202605
• N/A

Test result

Approach

What is the motivation for this PR?

How did you do it?

How did you verify/test it?

Tested on LMA and UMA testbeds internally

https://elastictest.org/scheduler/testplan/6a7650e76fff0a820f1b2e53
https://elastictest.org/scheduler/testplan/6a7650be1b14326709d1077f

Any platform specific information?

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

Documentation

[mssonicbld]

/azp run

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[mssonicbld]

This PR has backport request label(s) for branch(es): msft-202603, but is missing required test information. Please make sure you tick the tested branch(es) in the Tested branch section and provide test evidence (e.g., 202603: <test result>) in the Test result section as well in your PR description.

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