refactor(integration-tests): simplify test harness

[RomanBrodetski]

Summary

• use reth fork that has a fix to properly release ports. Agreed with @itegulov on this. PR against main repo: fix(discv5): use Weak reference in kbuckets bg task to release port on shutdown paradigmxyz/reth#23282 (unfortunately no acknowledgement from maintainers for now)
• refactor the integration test harness around explicit environment -> config -> launch flow
• unify node launch around config derivation for external nodes and restarts
• trim redundant harness API surface and simplify gateway/multi-node ownership

Testing

• cargo test -p zksync_os_integration_tests --no-run
• cargo nextest run -p zksync_os_integration_tests --profile no-pig

[github-actions]

Test results

308 tests   308 ✅  32m 57s ⏱️
 24 suites    0 💤
  1 files      0 ❌

Results for commit 224bc73.

♻️ This comment has been updated with latest results.