Parse YAML respawn delay strings

[Old-Ding]

Description

The YAML frontend accepts strings for attributes declared as float, but ExecuteProcess.parse() compared a string-valued respawn_delay with 0.0 before converting it. A launch file using respawn_delay: '2.0' therefore failed during parsing with TypeError: '<' not supported between instances of 'str' and 'float'.

Convert string-valued respawn delays at the existing frontend parsing boundary. Valid values reach ExecuteLocal as floats, while invalid strings fail immediately with a descriptive ValueError. Native float behavior and the runtime respawn state machine are unchanged.

Is this user-facing behavior change?

Yes. Quoted numeric respawn_delay values in YAML launch files now work consistently instead of failing with a comparison TypeError. Invalid strings produce a parse-time error.

Did you use Generative AI?

Yes. OpenAI Codex assisted with tracing the frontend data flow, preparing the focused fix and tests, and reviewing the result. I reviewed the diff and verification output.

Additional Information

Local verification:

• reproduced the Rolling failure with respawn_delay: '2.0'
• all 279 launch functional tests passed
• all 18 launch_yaml functional tests passed
• both package flake8 tests passed
• targeted ament_mypy passed with zero errors
• ament_flake8, ament_pep257, and ament_copyright passed for both changed files
• compileall and git diff --check passed

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