onlineddl: e2e test for REVERT VITESS_MIGRATION completing on both shards

[shlomi-noach]

This PR adds a test to a behavior we know is correct, and is true in general for all ApplySchema operation, but just making it very explicit now.

Adds a follow-up sub-test to TestVreplSchemaChanges (the only e2e suite that runs on a 2-shard cluster) covering the happy path for revert: a migration completes on both shards, a REVERT VITESS_MIGRATION is issued via vtctldclient, and the revert completes successfully on both shards.

The existing sibling test ("Revert a migration completed on one shard and cancelled on another") covers the failure case where revert is impossible on one shard. This PR adds the complementary success case.

Verification

• WaitForMigrationStatus uses the full shards slice — the wait only returns when both shards reach terminal status
• ReadMigrations asserts exactly 2 rows, both complete, with the shard name surfaced in any failure message

[vitess-bot]

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[Copilot]

Pull request overview

Adds an end-to-end “happy path” coverage to the existing 2-shard OnlineDDL VReplication e2e suite to ensure REVERT VITESS_MIGRATION completes successfully on both shards when the original migration completed on both shards.

Changes:

• Add a new subtest that runs an OnlineDDL migration to completion on both shards.
• Issue REVERT VITESS_MIGRATION via vtctldclient ApplySchema and wait for completion across both shards.
• Assert SHOW VITESS_MIGRATIONS returns exactly two rows (one per shard) and both are complete for the revert UUID.