feat(admin): expose rebalance cleanup warnings

[cxymds]

Summary

• Add cleanupWarnings to rebalance pool status models and output schema.
• Surface cleanup warnings in human-readable rebalance status output without treating them as pool failures.
• Cover new and legacy backend response shapes in unit and command tests.

Testing

• cargo fmt --all
• cargo fmt --all --check
• cargo clippy --workspace -- -D warnings
• cargo test --workspace

Notes

• Compatible with backend responses that omit cleanupWarnings; the field defaults to zero warnings.