[Backport 2025.4] fix(nemesis): skip disrupt_refuse_connection nemesis when target is alone in rack

[scylladbbot]

The nemeses disrupt_refuse_connection_with_block_scylla_ports_on_banned_node and disrupt_refuse_connection_with_send_sigstop_signal_to_scylla_on_banned_node` need to remove the target node at some point, and if that node is the only one in the rack, that is not allowed.

Example of failure:
https://argus.scylladb.com/tests/scylla-cluster-tests/f4267f80-cf89-4611-8d74-58f0592f33f9

2026-03-11 00:14:39.759: (DisruptionEvent Severity.ERROR) period_type=end event_id=d15192fb-c14d-4a82-903e-d67140a1cfc2 duration=6m42s: nemesis_name=IsolateNodeWithIptableRuleNemesis target_node=Node elasticity-test-nemesis-master-db-node-f4267f80-3 [18.203.101.143 | 10.4.2.41] (Type: i8g.large) (rack: RACK2) errors=Node was not removed properly (Node status:{'state': 'DN', 'load': '381.20GB', 'tokens': '256', 'owns': '?', 'host_id': 'be4a0f28-65a5-47b0-a922-cf9a5df22211', 'rack': 'RACK2'})
Traceback (most recent call last):
  File "/home/ubuntu/scylla-cluster-tests/sdcm/nemesis/__init__.py", line 6084, in _refuse_connection_from_banned_node
    working_node.run_nodetool(f"removenode {target_host_id}", retry=0, long_running=True)
    ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/ubuntu/scylla-cluster-tests/sdcm/cluster.py", line 3100, in run_nodetool
    result = runner(cmd, timeout=timeout, ignore_status=ignore_status, verbose=verbose, retry=retry)
  File "/home/ubuntu/scylla-cluster-tests/sdcm/remote/remote_long_running.py", line 70, in run_long_running_cmd
    raise UnexpectedExit(result=result)
sdcm.remote.libssh2_client.exceptions.UnexpectedExit: Encountered a bad command exit code!
Command: '/usr/bin/nodetool  removenode be4a0f28-65a5-47b0-a922-cf9a5df22211 '
Exit code: 4
Stdout:
Stderr:
error executing POST request to http://localhost:10000/storage_service/remove_node with parameters {"host_id": ["be4a0f28-65a5-47b0-a922-cf9a5df22211"]}: remote replied with status code 500 Internal Server Error:
std::runtime_error (Removenode failed: node remove rejected: Cannot remove the node because its removal would make some existing keyspace RF-rack-invalid)
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File "/home/ubuntu/scylla-cluster-tests/sdcm/nemesis/__init__.py", line 6344, in wrapper
    result = method(*args, **kwargs)
  File "/home/ubuntu/scylla-cluster-tests/sdcm/nemesis/monkey/__init__.py", line 987, in disrupt
    self.runner.disrupt_refuse_connection_with_block_scylla_ports_on_banned_node()
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
  File "/home/ubuntu/scylla-cluster-tests/sdcm/nemesis/__init__.py", line 5982, in disrupt_refuse_connection_with_block_scylla_ports_on_banned_node
    self._refuse_connection_from_banned_node(use_iptables=True)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^
  File "/home/ubuntu/scylla-cluster-tests/sdcm/nemesis/__init__.py", line 6034, in _refuse_connection_from_banned_node
    ExitStack() as stack,
    ~~~~~~~~~^^
  File "/usr/local/lib/python3.14/contextlib.py", line 619, in __exit__
    raise exc
  File "/usr/local/lib/python3.14/contextlib.py", line 604, in __exit__
    if cb(*exc_details):
       ~~^^^^^^^^^^^^^^
  File "/home/ubuntu/scylla-cluster-tests/sdcm/nemesis/__init__.py", line 6043, in _finalizer
    self._remove_node_add_node(
    ~~~~~~~~~~~~~~~~~~~~~~~~~~^
        verification_node=working_node,
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        node_to_remove=self.target_node,
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        remove_node_host_id=target_host_id,
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/home/ubuntu/scylla-cluster-tests/sdcm/nemesis/__init__.py", line 4126, in _remove_node_add_node
    assert removed_node_status is None, "Node was not removed properly (Node status:{})".format(removed_node_status)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: Node was not removed properly (Node status:{'state': 'DN', 'load': '381.20GB', 'tokens': '256', 'owns': '?', 'host_id': 'be4a0f28-65a5-47b0-a922-cf9a5df22211', 'rack': 'RACK2'})

Testing

• [ ]

PR pre-checks (self review)

• I added the relevant backport labels
• I didn't leave commented-out/debugging code

Reminders

• Add New configuration option and document them (in sdcm/sct_config.py)

• Add unit tests to cover my changes (under unit-test/ folder)

• Update the Readme/doc folder relevant to this change (if needed)

• (cherry picked from commit c0656a6)

Parent PR: #13965

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Code review

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