[critical_services] Exclude snmp from tracking list on BMC topology

[Gfrom2016]

Description of PR

Summary:
Exclude snmp from the critical-services tracking list when the DUT is running on a BMC topology.

On a SONiC BMC board (and BMC topologies in general) the BMC runs a management-plane SONiC image. SNMP is not deployed on BMC by design ΓÇö it runs on the host NOS, not on the BMC. As a result, the default critical_services = [pmon, snmp, database, lldp] tracking list is incorrect for BMC, and a wide range of tests fail during teardown/sanity with:

Failed: All critical services should be fully started!

with the final snapshot being {'pmon': True, 'snmp': False, 'database': True, 'lldp': True}.

This PR mirrors the existing DPU pattern (a few lines above, introduced for the same reason on DPU) and removes snmp from the tracking list when 'bmc' in topo_type.

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?

Unblock ~20 test failures on the SONiC BMC testbed whose only failure mode is the framework tracking a service (snmp) that is not deployed on SONiC BMC. Tests affected (all failed on teardown / critical process check with the same snmp: False footprint):

Feature	File	Cases
dns	dns/static_dns/test_static_dns.py	1
dns	dns/test_dns_resolv_conf.py	1
generic_config_updater	generic_config_updater/test_cacl.py	8
platform_tests	platform_tests/test_reboot.py	1
syslog	syslog/test_logrotate.py	1
container_hardening	container_hardening/test_container_hardening.py	1
db_migrator	db_migrator/test_migrate_dns.py	1
golden_config_infra	golden_config_infra/test_config_reload_with_rendered_golden_config.py	1
override_config_table	override_config_table/test_override_config_table.py	1
scp	scp/test_scp_copy.py	1
tacacs	tacacs/test_authorization.py	2

How did you do it?

In MultiAsicSonicHost.critical_services_tracking_list(), added a BMC guard right after the existing DPU guard:

is_bmc = 'bmc' in (self.topo_type or '')
if is_bmc and 'snmp' in service_list:
    service_list.remove('snmp')

No behavioral change for non-BMC topologies.

How did you verify/test it?

Verified on a SONiC BMC DUT (testbed-host1-komodo-bmc, Komodo BMC board) by applying the patch locally in the sonic-mgmt container and re-running one of the affected tests, syslog/test_logrotate.py::test_logrotate_small_size.

• Before the patch: test failed with Failed: All critical services should be fully started!, snapshot = {'pmon': True, 'snmp': False, 'database': True, 'lldp': True}.
• After the patch: the critical_services_fully_started check now passes (framework-tracked services list becomes {pmon, database, lldp}, matching what is actually deployed on BMC); execution proceeds past that check.

Any platform specific information?

Only affects DUTs whose topo_type contains bmc.

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

[mssonicbld]

/azp run

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