fix(nemesis): handle new keyspace being written to during AddRemoveDCNemesis

[cezarmoise]

If a new keyspace is created and written to by cassandra-stress while there is another DC, because the stress command only specifies replication_factor=3, it will try to write to that DC and get a schema creation failure.

• change the number of nodes in the new DC to 3 (matches RF in most commands)
• look for keyspaces from outside the nemesis, and revert their RF if needed.

Closes: SCT-130

Testing

• https://argus.scylladb.com/tests/scylla-cluster-tests/196a37b7-f5a4-4dc8-8f50-35f0b33eca26
  Tested on another branch were I added a nemesis that runs in parallel and just creates and writes to some keyspaces

From sct.log, 4 instances where the keyspace created by the second nemesis was altered to have RF=0 on the new DC.

2026-08-14 10:48:48,990 ... Executing CQL 'ALTER KEYSPACE system_distributed WITH replication = {'class': 'NetworkTopologyStrategy', 'eu-west-1': 3}' ...
2026-08-14 10:49:10,761 ... Executing CQL 'ALTER KEYSPACE system_traces WITH replication = {'class': 'NetworkTopologyStrategy', 'eu-west-1': 2}' ...
2026-08-14 10:49:32,812 ... Executing CQL 'ALTER KEYSPACE system_distributed WITH replication = {'class': 'NetworkTopologyStrategy', 'eu-west-1': 3, 'add_remove_nemesis_dc': 3}' ...
2026-08-14 10:49:55,263 ... Executing CQL 'ALTER KEYSPACE system_traces WITH replication = {'class': 'NetworkTopologyStrategy', 'eu-west-1': 2, 'add_remove_nemesis_dc': 3}' ...
2026-08-14 10:50:02,385 ... Executing CQL 'ALTER KEYSPACE keyspace_new_dc WITH replication = {'class': 'NetworkTopologyStrategy', 'eu-west-1': ['RACK1', 'RACK2', 'RACK0'], 'add_remove_nemesis_dc': 3}' ...
2026-08-14 11:08:59,209 ... Executing CQL 'ALTER KEYSPACE system_distributed WITH replication = {'class': 'NetworkTopologyStrategy', 'eu-west-1': 3, 'add_remove_nemesis_dc': 0}' ...
2026-08-14 11:09:18,289 ... Executing CQL 'ALTER KEYSPACE system_traces WITH replication = {'class': 'NetworkTopologyStrategy', 'eu-west-1': 2, 'add_remove_nemesis_dc': 0}' ...
2026-08-14 11:09:23,177 ... Executing CQL 'ALTER KEYSPACE keyspace_new_dc WITH replication = {'class': 'NetworkTopologyStrategy', 'eu-west-1': ['RACK1', 'RACK2', 'RACK0'], 'add_remove_nemesis_dc': 0}' ...
2026-08-14 11:10:48,728 ... Executing CQL 'ALTER KEYSPACE "keyspace_stress_write_SFM9ZJ8D" WITH replication = {'class': 'NetworkTopologyStrategy', 'add_remove_nemesis_dc': 0, 'eu-west-1': ['RACK1', 'RACK2', 'RACK0']}' ...
2026-08-14 11:11:56,717 ... Executing CQL 'ALTER KEYSPACE "keyspace_stress_write_F5EDKP0N" WITH replication = {'class': 'NetworkTopologyStrategy', 'add_remove_nemesis_dc': 0, 'eu-west-1': ['RACK1', 'RACK2', 'RACK0']}' ...
2026-08-14 11:19:49,829 ... Executing CQL 'ALTER KEYSPACE system_distributed WITH replication = {'class': 'SimpleStrategy', 'replication_factor': 3}' ...
2026-08-14 11:19:53,403 ... Executing CQL 'ALTER KEYSPACE system_traces WITH replication = {'class': 'SimpleStrategy', 'replication_factor': 2}' ...
2026-08-14 11:30:39,678 ... Executing CQL 'ALTER KEYSPACE system_distributed WITH replication = {'class': 'NetworkTopologyStrategy', 'eu-west-1': 3}' ...
2026-08-14 11:31:00,542 ... Executing CQL 'ALTER KEYSPACE system_traces WITH replication = {'class': 'NetworkTopologyStrategy', 'eu-west-1': 2}' ...
2026-08-14 11:31:07,602 ... Executing CQL 'ALTER KEYSPACE system_distributed WITH replication = {'class': 'NetworkTopologyStrategy', 'eu-west-1': 3, 'add_remove_nemesis_dc': 3}' ...
2026-08-14 11:31:30,574 ... Executing CQL 'ALTER KEYSPACE system_traces WITH replication = {'class': 'NetworkTopologyStrategy', 'eu-west-1': 2, 'add_remove_nemesis_dc': 3}' ...
2026-08-14 11:31:38,952 ... Executing CQL 'ALTER KEYSPACE keyspace_new_dc WITH replication = {'class': 'NetworkTopologyStrategy', 'eu-west-1': ['RACK1', 'RACK2', 'RACK0'], 'add_remove_nemesis_dc': 3}' ...
2026-08-14 11:48:51,538 ... Executing CQL 'ALTER KEYSPACE system_distributed WITH replication = {'class': 'NetworkTopologyStrategy', 'eu-west-1': 3, 'add_remove_nemesis_dc': 0}' ...
2026-08-14 11:49:11,815 ... Executing CQL 'ALTER KEYSPACE system_traces WITH replication = {'class': 'NetworkTopologyStrategy', 'eu-west-1': 2, 'add_remove_nemesis_dc': 0}' ...
2026-08-14 11:49:30,158 ... Executing CQL 'ALTER KEYSPACE keyspace_new_dc WITH replication = {'class': 'NetworkTopologyStrategy', 'eu-west-1': ['RACK1', 'RACK2', 'RACK0'], 'add_remove_nemesis_dc': 0}' ...
2026-08-14 11:50:08,799 ... Executing CQL 'ALTER KEYSPACE "keyspace_stress_write_S6CZ3IP8" WITH replication = {'class': 'NetworkTopologyStrategy', 'add_remove_nemesis_dc': 0, 'eu-west-1': ['RACK0', 'RACK2', 'RACK1']}' ...
2026-08-14 11:50:42,517 ... Executing CQL 'ALTER KEYSPACE "keyspace_stress_write_UC3DSMMF" WITH replication = {'class': 'NetworkTopologyStrategy', 'add_remove_nemesis_dc': 0, 'eu-west-1': ['RACK1', 'RACK2', 'RACK0']}' ...
2026-08-14 11:51:13,574 ... Executing CQL 'ALTER KEYSPACE system_distributed WITH replication = {'class': 'SimpleStrategy', 'replication_factor': 3}' ...
2026-08-14 11:51:17,417 ... Executing CQL 'ALTER KEYSPACE system_traces WITH replication = {'class': 'SimpleStrategy', 'replication_factor': 2}' ...

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)

[scylladb-promoter]

✅ Test Summary: PASSED

✅ Precommit: PASSED

Total	Passed	Failed	Skipped
28	16	0	12

✅ Tests: PASSED

Total	Passed	Failed	Errors	Skipped
4790	4759	0	0	31

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