fix: test a current generated union

[ebarroca]

Summary

• replace the removed document-table union fixture with the current data-store schema union
• preserve coverage that known union variants tolerate unknown response fields
• assert the concrete generated union variant

Test Plan

• mvn -B test
• mvn -B spotless:check
• mvn -B verify
• regenerated the client from the latest upstream OpenAPI contract with OpenAPI Generator 7.22.0 and ran mvn -B clean test