basalt-26.3: sqlproxy: TestConnectionMigration failed

sqlproxy.TestConnectionMigration [failed](https://mesolite.cluster.engflow.com/invocations/default/0083ad9b-b893-4d49-bded-2fb970027f5d?testReportRun=6&testReportShard=4&testReportAttempt=1#targets-Ly9wa2cvc3FscHJveHk6c3FscHJveHlfdGVzdA==) on basalt-26.3 @ [1326f194d41c7a5187da0509cee33d331d2229de](https://github.com/cockroachdb/cockroach/commits/1326f194d41c7a5187da0509cee33d331d2229de):

```
=== RUN TestConnectionMigration
test_log_scope.go:172: test logs captured to: outputs.zip/logTestConnectionMigration3899560322
test_log_scope.go:82: use -show-logs to present logs inline
proxy_handler_test.go:2397: -- test log scope end --
test logs left over in: outputs.zip/logTestConnectionMigration3899560322
--- FAIL: TestConnectionMigration (23.21s)
=== RUN TestConnectionMigration/transfer_timeout_in_response
proxy_handler_test.go:2371:
Error Trace: pkg/sqlproxy/proxy_handler_test.go:2371
Error: An error is expected but got nil.
Test: TestConnectionMigration/transfer_timeout_in_response
--- FAIL: TestConnectionMigration/transfer_timeout_in_response (10.02s)
```

Parameters:
- attempt=1
- run=6
- shard=4
Help

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

/cc @cockroachlabs/sqlproxy-prs

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

Jira issue: CRDB-67704

Start with pkg/sqlproxy/proxy_handler_test.go, especially TestConnectionMigration and the transfer_timeout_in_response subtest around line 2371. Run the failing test and inspect the linked test report and captured logs to determine why no error was returned; done means the failure is understood, the appropriate behavior or test expectation is corrected, and TestConnectionMigration passes reliably.

Written by the indexing model from the issue text.