release-26.2.3-rc: pkg/upgrade/upgrademanager/upgrademanager_test: TestMigrationFailure failed

pkg/upgrade/upgrademanager/upgrademanager_test.TestMigrationFailure [failed](https://mesolite.cluster.engflow.com/invocations/default/9016085f-2287-425e-8d0f-3f882183e322?testReportRun=2&testReportShard=1&testReportAttempt=1#targets-Ly9wa2cvdXBncmFkZS91cGdyYWRlbWFuYWdlcjp1cGdyYWRlbWFuYWdlcl90ZXN0) on release-26.2.3-rc @ [90d3b6080727dc810224a99903cb2e88b81e91ae](https://github.com/cockroachdb/cockroach/commits/90d3b6080727dc810224a99903cb2e88b81e91ae):

```
=== RUN TestMigrationFailure
test_log_scope.go:172: test logs captured to: outputs.zip/logTestMigrationFailure4187743498
test_log_scope.go:82: use -show-logs to present logs inline
manager_external_test.go:857: test will fail at version: 25.4-upgrading-to-26.1-step-004
test_server_shim.go:638: DRPC is enabled (via testserver factory defaults)
```

Parameters:
- attempt=1
- race=true
- run=2
- shard=1
Help

See also: [How To Investigate a Go Test Failure \(internal\)](https://cockroachlabs.atlassian.net/l/c/HgfXfJgM)

/cc @cockroachlabs/release-eng @cockroachlabs/release-eng

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Jira issue: CRDB-65128

Start with pkg/upgrade/upgrademanager/upgrademanager_test and TestMigrationFailure, reproducing the failure with race enabled at the reported release commit. Inspect the linked test report and captured outputs.zip logs around version 25.4-upgrading-to-26.1-step-004. Done means the migration failure is understood and TestMigrationFailure passes reliably under the reported parameters.

Written by the indexing model from the issue text.