Add BGP local-preference test cases for T1 topology

[bpar9]

Description of PR

Summary:
Adds tests/bgp/test_bgp_local_preference.py implementing 21 test cases for BGP local-preference functionality on T1 topology, based on the BGP Local-Preference Test Plan.

Test cases cover:

1. Default local-preference value (100) for eBGP-learned routes
2. Setting local-preference via inbound route-map
3. Best path selection preferring higher local-preference
4. Local-preference propagation to iBGP peers
5. Local-preference NOT sent to eBGP peers
6. Persistence after BGP restart
7. IPv6 route support
8. Boundary values (0 and 4294967295)
9. Local-preference override via route-map update
10. Multiple prefixes with same local-preference
11. Multiple paths (3+ neighbors) with different values
12. Tie-breaking with equal local-preference (AS-path secondary)
13. Local-preference takes precedence over AS-path length
14. Local-preference takes precedence over MED
15. Interaction with weight attribute
16. Route flapping stability
    17–21. Route reflector scenarios (preservation, setting, best path, multiple clients, client override)

Link to Devin run: https://cisco-demo.devinenterprise.com/sessions/5d7e6751313845458da92a5214eca3b4
Requested by: @bpar9

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?

Provide automated test coverage for BGP local-preference attribute behavior, including route-map configuration, best path selection, iBGP/eBGP propagation rules, interaction with other path attributes (AS-path, MED, weight), and route reflector scenarios.

How did you do it?

• Single new file: tests/bgp/test_bgp_local_preference.py
• Uses ExaBGP on PTF host to announce/withdraw test routes via HTTP API
• Uses vtysh commands to configure route-maps and query BGP RIB (JSON output)
• Module-scoped setup fixture gathers topology info (T0/T2 neighbors, ASN, ExaBGP ports, next-hops)
• Autouse cleanup fixture ensures route-map removal after each test
• Helper functions for ExaBGP interaction, route-map config, BGP route queries

How did you verify/test it?

• Passes flake8 lint checks
• Note: Not tested on actual testbed - requires T1 topology with ExaBGP running on PTF host

Any platform specific information?

None - uses standard FRR/vtysh commands

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

T1 topology only (@pytest.mark.topology('t1'))

Human Review Checklist

Reviewers should verify:

• ExaBGP HTTP API pattern (POST to http://<ptfip>:<port> with commands field) matches testbed setup
• vtysh JSON output structure (paths, locPrf, bestpath, peerId, aspath) matches FRR version
• T0 neighbors = iBGP peers, T2 neighbors = eBGP peers assumption is correct for T1
• Test prefixes (10.100.0.0/24, 2001:db8:100::/64, 10.200.x.0/24) don't conflict with testbed routes
• Route reflector config/cleanup in tests 17-21 is safe for the testbed
• Variable naming (shortened for 79-char compliance) is acceptable for maintainability

Documentation

N/A - test code only

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