test: decouple inject skip coverage from mysql cdc reset test

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Summary

• move inject-source-offsets skip-forward behavior coverage to the cron-only Kafka alter test path
• keep ci/scripts/e2e-source-mysql-cdc-reset.sh focused on reset/binlog-expiry recovery only
• harden cron-only CI stability by waiting for service readiness in script and using compose health conditions

What changed

• e2e_test/source_inline/kafka/alter/cron_only/alter_source_properties_safe.slt.serial
  • add forward-skip verification for inject-source-offsets
  • keep rewind/re-consume verification
  • use a single-partition topic for deterministic count assertions
• e2e_test/source_inline/kafka/alter/get_kafka_offsets.py
  • add --skip-next mode for forward-skip injection
  • keep --skip-last for rewind behavior
• ci/scripts/e2e-source-cron-only-test.sh
  • add explicit readiness waits for Postgres and Kafka before starting risedev
• ci/docker-compose.yml
  • use service_healthy conditions for source-cron-only-test-env dependencies
• ci/scripts/e2e-source-mysql-cdc-reset.sh
  • remove inject/offset-internal coupling
  • keep only reset-recovery scenario and boundary assertions

Why

This separates two test responsibilities clearly:

1. unsafe offset injection semantics (rewind/skip) in cron-only Kafka tests
2. MySQL CDC reset recovery semantics in the reset script

This improves signal quality and reduces flaky coupling between unrelated behavior checks.