[bgp] Use correct DUT MAC for BGP monitor peers

[micasyang]

Description of PR

Summary:

Fix BGP monitor sessions on platforms using per-port MAC addresses.
The global router_mac may differ from the MAC used by the selected
DUT interface, causing ExaBGP response traffic to use an incorrect
destination MAC.

Type of change

• Bug fix
• Testbed and Framework(new/improvement)
• New Test case
• Test case improvement

Back port request

N/A — this PR directly targets the 202603 branch.

Approach

What is the motivation for this PR?

The following tests fail because the BGP monitor session cannot be
established:

• bgp/test_bgpmon.py::test_bgpmon
• bgp/test_traffic_shift.py::test_load_minigraph_with_traffic_shift_away

Observed error:

BGP monitor session is not established

How did you do it?

• Use the selected L3 interface MAC in the traffic-shift BGP monitor fixture.
• Use the captured DUT SYN source MAC in test_bgpmon.

How did you verify/test it?

Before the fix, both failures were reproduced on a physical c0 testbed.

After the fix, both affected tests passed on a physical c0 testbed.

Any platform specific information?

This issue affects platforms using per-port MAC addresses.

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

N/A

Documentation

N/A

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