Fix flaky topic validation caused by intermittent Topicana empty response

[arizz96]

Summary

• The Legacy Kafka ACL validation service (Topicana) occasionally returns an empty string instead of actual ACL data
• When this happens, the platform binary treats all ACLs as invalid and fails the CI job
• Wraps the build.sh topic_validation call in a retry loop (3 attempts, 15s delay) so transient Topicana outages don't fail the CI check
• Topic config generation is unaffected — it only runs once before the retry loop

Test plan

• Trigger the validate-topics CI job in a repo like fadmin and confirm it passes
• If Topicana returns empty on first attempt, verify the retry fires after 15s and succeeds on retry

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