flaky test: SpannerRepositoryIntegrationTests.testLoadsCorrectData

Failed at 5.x release PR
https://github.com/GoogleCloudPlatform/spring-cloud-gcp/actions/runs/17080355816/job/48432413763?pr=4082
```
Error: 5:897 [ERROR] SpannerRepositoryIntegrationTests.testLoadsCorrectData:131
Expecting actual:
["demo_trader_json3", "demo_trader_json4"]
to contain exactly in any order:
["demo_trader1",
"demo_trader2",
"demo_trader3",
"demo_trader_json1",
"demo_trader_json2",
"demo_trader_json3",
"demo_trader_json4"]
but could not find the following elements:
["demo_trader1",
"demo_trader2",
"demo_trader3",
"demo_trader_json1",
"demo_trader_json2"]
```

Start by running SpannerRepositoryIntegrationTests.testLoadsCorrectData and compare its setup with the failure from the 5.x release PR. Trace why only demo_trader_json3 and demo_trader_json4 are loaded, then make the test reliably verify the complete expected data set.

Written by the indexing model from the issue text.