[action] [PR:20125] Handle Multi-port portchannel member case in ECMP member flap TC

[mssonicbld]

Description of PR

Summary:
Fixes #17887

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

Approach

What is the motivation for this PR?

This fixes issue mentioned in #17887.
For pizza box DUTs, multi-port portchannel scenario was not handled, which caused issue on certain specific topologies.

How did you do it?

Add all members of PORT_CHANNEL (if the port being shutdown is part of PORT_CHANNEL) to the filtered ports list, so that during fib_test, these ports are not randomly picked as src port.

How did you verify/test it?

Verified on t1-lag topology. the testcase passes.
Logs:

07/08/2025 20:31:13 test_fib.test_ecmp_group_member_flap L0864 INFO | Shutting down one uplink port.
07/08/2025 20:31:13 test_fib.test_ecmp_group_member_flap L0869 INFO | Shutting down port Ethernet0
07/08/2025 20:31:31 test_fib.get_port_and_portchannel_member L0764 INFO | Port Ethernet0 is a member of port channel PortChannel102, adding all members: ['Ethernet0', 'Ethernet8']
07/08/2025 20:31:31 test_fib.get_port_and_portchannel_member L0773 INFO | Found PTF port 0 for port channel member Ethernet0
07/08/2025 20:31:31 test_fib.get_port_and_portchannel_member L0773 INFO | Found PTF port 1 for port channel member Ethernet8
07/08/2025 20:31:31 test_fib.test_ecmp_group_member_flap L0881 INFO | Verifying ECMP behavior after member down.
["/root/env-python3/bin/ptf --test-dir ptftests/py3 fib_test.FibTest --platform-dir ptftests --qlen=20000 --platform remote -t 'fib_info_files=['\"'\"'/root/fib_info_dut_test_ecmp_group_member_flap[True-False-1514]_0.txt'\"'\"'];
ptf_test_port_map='\"'\"'/root/ptf_test_port_map.json'\"'\"';ipv4=True;ipv6=False;testbed_mtu=1514;test_balancing=True;ignore_ttl=False;single_fib_for_duts='\"'\"'multiple-fib'\"'\"';switch_type='\"'\"''\"'\"';;
skip_src_ports=[**0, 1]**' --relax --debug info --log-file /tmp/fib_test.ecmp_member_flap.member_down.ipv4.True.ipv6.False.2025-08-07-20:26:22.log --socket-recv-size 16384"], kwargs={"chdir": "/root", "module_ignore_errors": false, "module_async": false}

Any platform specific information?

NA

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

NA

Documentation

[mssonicbld]

Original PR: #20125

[mssonicbld]

/azp run

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