KAFKA-16109: Add system test for simple consumer commit and restart

[Shally-Katariya]

Motivation

Kafka currently lacks a focused system test that validates a simple
end-to-end consumer workflow with manual offset commits across a restart.

Changes

• Added a system test extending the existing produce-consume validation
  template.
• The test restarts a manually committing consumer mid-run and verifies
  that consumption resumes from committed offsets without duplicate delivery.

Notes

• Reuses existing Kafka system test infrastructure.
• No production code changes.