CI: Expanded failover scenario tests

[renecannao]

Summary

The current failover tests cover graceful master takeover and hard failover (DeadMaster). Several important failover scenarios are untested:

• Intermediate master failure (chain topology)
• Errant GTID detection and handling
• Co-master failover

Proposal

• Intermediate master failure: Set up chain topology (mysql1→mysql2→mysql3), kill mysql2, verify mysql3 is relocated under mysql1
• Errant GTID: Inject an errant transaction on a replica, verify orchestrator detects it via /api/errant-gtid
• Co-master failover: Set up co-master pair, kill one, verify recovery

Acceptance Criteria

• Chain topology test: intermediate master failure + replica relocation
• Errant GTID: injection, detection via API, and locate-gtid-errant
• Co-master: setup, failure, and recovery
• All tests integrated into CI workflow

[coderabbitai]

🔗 Related PRs

#51 - Add functional test CI: Docker MySQL topology + ProxySQL + failover tests [merged]

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