kv/kvserver: TestPrepareMergeManifestsRHSFlushedEmpty failed

kv/kvserver.TestPrepareMergeManifestsRHSFlushedEmpty [failed](https://teamcity.cockroachdb.com/buildConfiguration/Cockroach_Ci_TestsAwsLinuxArm64_UnitTests/21522409?buildTab=log) with [artifacts](https://teamcity.cockroachdb.com/buildConfiguration/Cockroach_Ci_TestsAwsLinuxArm64_UnitTests/21522409?buildTab=artifacts#/) on master @ [fd02e41cebf8477b9be242aefb88745447d85f95](https://github.com/cockroachdb/cockroach/commits/fd02e41cebf8477b9be242aefb88745447d85f95):

```
=== RUN TestPrepareMergeManifestsRHSFlushedEmpty
test_log_scope.go:172: test logs captured to: /artifacts/tmp/_tmp/1a39945979bd2ca5a0139df86f748a97/logTestPrepareMergeManifestsRHSFlushedEmpty3693840364
test_log_scope.go:82: use -show-logs to present logs inline
client_merge_manifest_test.go:208: -- test log scope end --
test logs left over in: /artifacts/tmp/_tmp/1a39945979bd2ca5a0139df86f748a97/logTestPrepareMergeManifestsRHSFlushedEmpty3693840364
--- FAIL: TestPrepareMergeManifestsRHSFlushedEmpty (49.35s)
=== RUN TestPrepareMergeManifestsRHSFlushedEmpty/pebble
test_server_shim.go:146: using test tenant configuration from testserver factory defaults
test_server_shim.go:637: DRPC is enabled (via testserver factory defaults)
client_merge_manifest_test.go:173: condition failed to evaluate within 45s: from client_merge_manifest_test.go:190: [n2,s2,r88/2:{/Table/Max-\xfasplit}]: manifest still lists 1 tables

goroutine dump: /artifacts/tmp/_tmp/1a39945979bd2ca5a0139df86f748a97/goroutine_dump_3977631430.txt
--- FAIL: TestPrepareMergeManifestsRHSFlushedEmpty/pebble (47.86s)
```
Help

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

/cc @cockroachlabs/kv-triage

[Improve this report!](https://github.com/cockroachdb/cockroach/tree/master/pkg/cmd/bazci/githubpost/issues)

Jira issue: CRDB-68498

Start with client_merge_manifest_test.go around lines 173-208 and reproduce TestPrepareMergeManifestsRHSFlushedEmpty, using the linked TeamCity artifacts and captured test logs for details. Investigate the Pebble subtest and the 45-second wait reported at line 190. Done means the test completes without timing out and the merge manifest no longer incorrectly lists one table.

Written by the indexing model from the issue text.