release-26.3.0-rc: sql/backfill: TestVectorIndexWithPrefixBackfill failed

sql/backfill.TestVectorIndexWithPrefixBackfill [failed](https://mesolite.cluster.engflow.com/invocations/default/24245e4b-02ce-40f7-9568-e95ee62df892?testReportRun=2&testReportShard=1&testReportAttempt=1#targets-Ly9wa2cvc3FsL2JhY2tmaWxsOmJhY2tmaWxsX3Rlc3Q=) on release-26.3.0-rc @ [0a3dc2524e335b85175c507a318dd57f28f92ef3](https://github.com/cockroachdb/cockroach/commits/0a3dc2524e335b85175c507a318dd57f28f92ef3):

```
=== RUN TestVectorIndexWithPrefixBackfill
test_log_scope.go:172: test logs captured to: outputs.zip/logTestVectorIndexWithPrefixBackfill2671578743
test_log_scope.go:82: use -show-logs to present logs inline
test_server_shim.go:168: automatically injected an external process virtual cluster under test; see comment at top of test_server_shim.go for details.
```

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)

Same failure on other branches

- #172226 sql/backfill: TestVectorIndexWithPrefixBackfill failed [C-test-failure O-robot T-specialized-indexing branch-master]
- #171808 release-26.3: sql/backfill: TestVectorIndexWithPrefixBackfill failed [C-test-failure O-robot T-specialized-indexing branch-release-26.3]

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

Jira issue: CRDB-66758

Start with the sql/backfill.TestVectorIndexWithPrefixBackfill failure report and captured logs at outputs.zip/logTestVectorIndexWithPrefixBackfill2671578743. Read the comment in test_server_shim.go about the injected external process virtual cluster, then reproduce the race-enabled failure on release-26.3.0-rc. Done means the test passes reliably and the underlying regression is covered by the test suite.

Written by the indexing model from the issue text.