skip bgp_prefix for isolated topo. (sonic-net#21962)

Description of PR
Summary:
Fix the following log analysis error when running generic_config_updater/test_bgp_prefix.py. it's due to the BGP template used for isolated topo is different.

2026 Jan 18 05:17:43.204062 xxxx ERR bgp#bgpcfgd: BGPAllowListMgr::Default action community value is not found. route-map 'ALLOW_LIST_DEPLOYMENT_ID_0_V4' entry. seq_no=65535
same test case has been skipped for 202412 in sonic-net#18612
skip it for 202505 for isolated topo

Type of change
 Bug fix
 Testbed and Framework(new/improvement)
 New Test case
 Skipped for non-supported platforms
 Test case improvement

Approach
What is the motivation for this PR?
Skip unsupported feature

How did you do it?
mark condition

How did you verify/test it?
local testbed

Signed-off-by: Dashuai Zhang <[email protected]>

Author: sdszhang

tests/common/plugins/conditional_mark/tests_mark_conditions.yaml

@@ -1513,6 +1513,7 @@ generic_config_updater/test_bgp_prefix.py::test_bgp_prefix_tc1_suite:
     conditions:
       - "platform in ['x86_64-8122_64eh_o-r0', 'x86_64-8122_64ehf_o-r0']"
       - "asic_type in ['vs'] and https://github.com/sonic-net/sonic-mgmt/issues/18445"
+      - "'isolated' in topo_name"
 
 generic_config_updater/test_dhcp_relay.py:
   skip: