release-26.3: sql/backfill: TestVectorIndexMergingDuringBackfill failed

sql/backfill.TestVectorIndexMergingDuringBackfill [failed](https://mesolite.cluster.engflow.com/invocations/default/ad5df088-ed0c-4b73-b478-6358b2f179ea?testReportRun=2&testReportShard=1&testReportAttempt=1#targets-Ly9wa2cvc3FsL2JhY2tmaWxsOmJhY2tmaWxsX3Rlc3Q=) on release-26.3 @ [7c781e6a339c41ff01ad15ac12212256c86f6b01](https://github.com/cockroachdb/cockroach/commits/7c781e6a339c41ff01ad15ac12212256c86f6b01):

```
=== RUN TestVectorIndexMergingDuringBackfill
test_log_scope.go:172: test logs captured to: outputs.zip/logTestVectorIndexMergingDuringBackfill2299872737
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=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

- #172652 release-26.3.0-rc: sql/backfill: TestVectorIndexMergingDuringBackfill failed [C-test-failure O-robot T-sql-queries branch-release-26.3.0-rc]

[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-66117

Start by reproducing sql/backfill.TestVectorIndexMergingDuringBackfill at commit 7c781e6a339c41ff01ad15ac12212256c86f6b01 with -show-logs, then inspect the captured outputs.zip logs and the test_server_shim.go guidance. Done means the cause is addressed and the test passes reliably on release-26.3 with the reported race, run, shard, and attempt parameters.

Written by the indexing model from the issue text.