Support choices lists for YAML launch arguments

[Old-Ding]

Description

YAML launch arguments currently support choices only through the XML-shaped form:

choice:
  - value: ur3
  - value: ur5e

Allow the natural YAML shorthand choices: [ur3, ur5e]. The YAML entity adapter validates the shorthand as a string list and maps it to the same choice/value entities already consumed by DeclareLaunchArgument.parse(). The existing syntax and common launch layer remain unchanged; providing both forms is still rejected by the existing unparsed-key validation.

Fixes #698.

Is this user-facing behavior change?

Yes. YAML launch files can now declare argument choices with a scalar list matching the Python API's choices spelling.

Did you use Generative AI?

Yes. OpenAI Codex assisted with root-cause analysis and preparing the focused code and regression test. I reviewed the diff and verification results.

Additional Information

Local verification:

• all 20 launch_yaml functional tests passed
• ament_flake8, ament_pep257, and ament_copyright passed for all 22 checked files
• regression tests verify legacy syntax, mixed-type rejection, and ambiguous old/new key rejection
• compileall and git diff --check passed
• ament_mypy reports the same 16 pre-existing Entity/Parser annotation errors on both the clean baseline and this branch, with no new diagnostics

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