Materialize TimerAction action iterables

[Old-Ding]

Summary

• materialize TimerAction.actions once at construction
• prevent introspection from consuming one-shot iterables before the timer fires
• cover repeated access after describing conditional sub-entities

Testing

• focused source behavior probe: baseline retains no actions after introspection, while the fixed implementation retains them
• launch/test/launch/test_timer_action.py: 7 passed
• ament_flake8, ament_pep257, and ament_copyright on both modified files
• python -m py_compile on both modified files
• git diff --check

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