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

sql/backfill.TestVectorIndexMergingDuringBackfill [failed](https://mesolite.cluster.engflow.com/invocations/default/bd20537a-acaa-49e4-81e8-836dc13ec79f?testReportRun=3&testReportShard=1&testReportAttempt=1#targets-Ly9wa2cvc3FsL2JhY2tmaWxsOmJhY2tmaWxsX3Rlc3Q=) on release-26.3.0-rc @ [0a3dc2524e335b85175c507a318dd57f28f92ef3](https://github.com/cockroachdb/cockroach/commits/0a3dc2524e335b85175c507a318dd57f28f92ef3):

```
=== RUN TestVectorIndexMergingDuringBackfill
test_log_scope.go:172: test logs captured to: outputs.zip/logTestVectorIndexMergingDuringBackfill861189187
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.
test_server_shim.go:639: DRPC is enabled (via testserver factory defaults)
```

Parameters:
- attempt=1
- race=true
- run=3
- 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

- #172850 release-26.3: sql/backfill: TestVectorIndexMergingDuringBackfill 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:.*TestVectorIndexMergingDuringBackfill.*&sort=title+created&display=lastcommented+project) | [Improve this report!](https://github.com/cockroachdb/cockroach/tree/master/pkg/cmd/bazci/githubpost/issues)

Jira issue: CRDB-66687

Start with sql/backfill.TestVectorIndexMergingDuringBackfill and its linked test report, using the recorded race=true, run=3, and shard=1 parameters. Compare the failure with #172850 and the release-26.3.0-rc commit. Done means identifying and fixing the test failure and confirming the test passes.

Written by the indexing model from the issue text.