Preserve iterable executable arguments

[Old-Ding]

Summary

• materialize Executable.arguments and each nested argument iterable once at construction
• build the command and expose the public property from the same stored values
• prevent one-shot outer or nested argument iterables from appearing empty after normalization

Testing

• focused runtime probe reproduced a nested generator producing the correct command token while leaving arguments empty before the fix
• launch/test/launch/test_executable.py: 8 passed
• ament_flake8, ament_pep257, and ament_copyright on both modified files
• targeted mypy on launch/launch/descriptions/executable.py
• python -m py_compile on both modified files
• git diff --check

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