Clean up split integration test packages and de-duplicate common code

[pkoutsovasilis]

After splitting up the various integration test suites (serverless, leak, k8s, ech), we should audit and clean up duplicated code across these packages.

Goals:

• Extract shared helpers and setup logic into a common internal test utility package.
• Remove redundant logic or redefinitions.
• Improve maintainability and reduce future friction when updating shared test patterns.