basalt-26.3.2-rc-20260908: crosscluster/logical/txnmode: TestTxnModeUniqueConstraintUpdate failed

crosscluster/logical/txnmode.TestTxnModeUniqueConstraintUpdate [failed](https://mesolite.cluster.engflow.com/invocations/default/154a6740-7ddc-4b24-b932-f3322ce63402?testReportRun=1&testReportShard=2&testReportAttempt=1#targets-Ly9wa2cvY3Jvc3NjbHVzdGVyL2xvZ2ljYWwvdHhubW9kZTp0eG5tb2RlX3Rlc3Q=) on basalt-26.3.2-rc-20260908 @ [807558e5e81666af257c811e925e4cae570abc28](https://github.com/cockroachdb/cockroach/commits/807558e5e81666af257c811e925e4cae570abc28):

```
=== RUN TestTxnModeUniqueConstraintUpdate
test_log_scope.go:172: test logs captured to: outputs.zip/logTestTxnModeUniqueConstraintUpdate2101734743
test_log_scope.go:82: use -show-logs to present logs inline
test_server_shim.go:168: automatically injected a shared process virtual cluster under test; see comment at top of test_server_shim.go for details.
```

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

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

Same failure on other branches

- #174410 basalt-26.3: crosscluster/logical/txnmode: TestTxnModeUniqueConstraintUpdate failed [A-cdc C-test-failure O-robot T-cdc branch-basalt-26.3 release-blocker]

/cc @cockroachlabs/cdc

[This test on roachdash](https://roachdash.crdb.dev/?filter=status:open%20t:.*TestTxnModeUniqueConstraintUpdate.*&sort=title+created&display=lastcommented+project) | [Improve this report!](https://github.com/cockroachdb/cockroach/tree/master/pkg/cmd/bazci/githubpost/issues)

Start with crosscluster/logical/txnmode.TestTxnModeUniqueConstraintUpdate and inspect the linked invocation and captured logs at outputs.zip/logTestTxnModeUniqueConstraintUpdate2101734743. Reproduce the failure with race=true, run=1, shard=2, and compare the result across the referenced branches. Done means identifying the failure cause and restoring a passing test.

Written by the indexing model from the issue text.