mark xfail on generic hash test for isolated topo (#18071)

What is the motivation for this PR?
Mark xfail for flaky hashing behavior when using a specific combination of hash field and algorithm, such as CRC-INNER_L4_DST_PORT-ipv4-ipv6-nvgre.

How did you do it?
Mark test_generic_test.py as xfail in mark conditions yaml file

How did you verify/test it?
Verified it in nightly test run.

Any platform specific information?
str4-sn5600-1

Supported testbed topology if it's a new test case?
t0-isolated-u16d16s1

Author: Janetxxx

tests/common/plugins/conditional_mark/tests_mark_conditions.yaml

@@ -1135,6 +1135,10 @@ hash/test_generic_hash.py::test_nexthop_flap:
       - "asic_gen == 'spc1'"
       - "asic_type in ['broadcom']"
       - https://github.com/sonic-net/sonic-mgmt/issues/13919
+  xfail:
+    reason: "Flaky hashing behavior using specific combination of hash field and algorithm."
+    conditions:
+      - "'isolated' in topo_name"
 
 hash/test_generic_hash.py::test_nexthop_flap[CRC-INNER_IP_PROTOCOL:
   skip: