sctest: fix three defects in Rollback/Pause cumulative stage discovery

[waterWang]

Summary

The shared-cluster refactor of the cumulative schema changer test suites (commit 15081f2cf3c) introduced three defects in pkg/sql/schemachanger/sctest/cumulative.go. Their combined effect is that a large portion of the generated TestRollback_* and TestPause_* tests have been silently skipping since the refactor, and once the skipping is fixed, two further defects cause failures for tests that then actually run.

Defect 1: Stage discovery is poisoned by setup statements (tests silently skip)

stageDiscovery.countStagesOnce recorded post-commit stage counts from the first plan the BeforeStage knob observed. Because the knob is installed at cluster creation, any setup statement that runs through the declarative schema changer (e.g. CREATE SEQUENCE, CREATE INDEX) plans with zero post-commit stages, locking in 0 and causing cumulativeTestForEachPostCommitStage to skip with "test case has no post-commit stages".

Fix: Replace countStagesOnce (first-plan-wins) with observe, which overwrites the counts on every observed plan until freeze is called after the discovery pass completes. The last-observed (job) plan therefore wins, and multi-statement test definitions are also enabled.

Defect 2: rollbackCase/pauseCase drop the database but never recreate it

Both drop the test database and re-run setup without recreating it. Definitions that implicitly use defaultdb re-run their setup against system and fail with pq: user root does not have CREATE privilege on database system. The Backup flavor already handles this correctly by executing cs.CreateDatabaseStmt after the drop.

Fix: Recreate the database via cs.CreateDatabaseStmt after the drop in both rollbackCase and pauseCase, matching the Backup behavior.

Defect 3: Rollback() stops its shared server via t.Cleanup, causing spurious "leaked stopper" failures

t.Cleanup runs after the generated caller's defer log.Scope(t).Close(t), so the log scope teardown observes the still-running server and fails the test with leaked stopper. Pause() uses a plain defer and does not have this problem.

Fix: Stop the Rollback server with defer instead of t.Cleanup, matching Pause().

How to verify

• dev test pkg/sql/schemachanger -f 'TestRollback_add_column$' -v — previously completed in ~2s having skipped with "test case has no post-commit stages"; now runs the rollback scenarios.
• dev test pkg/sql/schemachanger -f 'TestRollback_alter_table_add_check_vanilla$' -v — previously failed with user root does not have CREATE privilege on database system and leaked stopper; now succeeds.

Closes #173199

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