fix: validate server group member quota before provisioning

[Dong Ma (larainema)]

Summary

• add a Nova limits preflight for the server_group_members quota
• fail cluster, node group, and scale-up requests before creating CAPI/CAPO resources when the requested server group size is too large
• include autoscaler max_node_count in the validation and return an actionable Magnum error

Testing

• /Users/dong/dongdev/VEXXHOST/magnum-cluster-api/.tox/unit/bin/python -m pytest magnum_cluster_api/tests/unit/test_utils.py -k 'server_group_members_quota' magnum_cluster_api/tests/unit/test_driver.py -k 'quota'
• uvx pre-commit run --files magnum_cluster_api/driver.py magnum_cluster_api/utils.py magnum_cluster_api/exceptions.py magnum_cluster_api/tests/conftest.py magnum_cluster_api/tests/unit/test_utils.py magnum_cluster_api/tests/unit/test_driver.py