--migrate: Testcontainers IT suite (base + edge cases)

Part of #37237. Depends on the Ledger & verification, Core migration engine, and CLI surface tickets.

Summary

Migration-specific integration tests (in dotCMS/dot-ca-event-manager), extending the existing Testcontainers harness (2 ClickHouse replicas + Keeper + Toxiproxy, per .claude/testing.md). May land as two PRs (base cases, then edge cases) even though it's tracked as one ticket.

Scope

Base cases
Seed via docker/init/*.sql, apply a candidate migration through the real runner, assert:

• pre-existing queries are unaffected
• new columns/defaults are correct
• the schema change propagated to the second replica (poll system.columns, the same shape as the existing assertReplicatedCount() helper)
• applying the same migration twice is a no-op

Edge cases — grounded in the real bug already caught once during design (mutations_sync=2 breaking retry-safety), not hypothetical:

• a metadata-only ADD COLUMN case
• a mutation-triggering case with mutation_id capture + poll to completion
• checksum-mismatch aborts the run
• V0-mismatch aborts the run
• two concurrent runners contend for the Keeper lock and only one proceeds
• a script interrupted mid-way is safe to re-run in full
• a dropped connection during a long mutation (Toxiproxy) doesn't cause a retry to enqueue a duplicate mutation
• a RENAME TABLE on a table with an attached refreshable MV, including one with a DEPENDS ON chain to another refreshable MV — three of the ten real environment-rollout runbooks (docs/migrations/) independently flag this as genuinely less battle-tested than a normal MV rename, not hypothetical caution

Verification (end-to-end, once this + all core tickets land)

Do a dry run of one real migration from the environment rollout (e.g. the simplest one, docs/migrations/07_engagement_daily.md) through the actual --migrate tool against a local stack, to confirm the tool works against genuine migration content, not just synthetic test SQL.

Repo

dotCMS/dot-ca-event-manager

Start with .claude/testing.md and the existing Testcontainers harness in dotCMS/dot-ca-event-manager, then inspect docker/init/*.sql and the assertReplicatedCount() helper. Implement the base and edge-case integration coverage described, including the real migration dry run using docs/migrations/07_engagement_daily.md once dependencies land. Done means the listed failure, retry, replication, locking, and refreshable-MV cases are verified.