fix: parse respawn_max_retries YAML strings

[Lidang-Jiang]

Summary

Parse YAML string values for respawn_max_retries into integers during frontend parsing, and reject empty strings and booleans with a clear ValueError.

Closes #968

Before

$ python direct_parse_respawn_max_retries.py  # upstream/rolling
"'0'" -> '0' (str)
"''" -> '' (str)
'true' -> True (bool)

After

$ git diff --check

$ python3 -m py_compile launch/launch/actions/execute_process.py launch_yaml/test/launch_yaml/test_executable.py
$WORKSPACE/python310_torch25_cuda/lib/python3.10/site-packages/xpytorch_import_hook.py:6: UserWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html. The pkg_resources package is slated for removal as early as 2025-11-30. Refrain from using this package or pin to Setuptools<81.
  import pkg_resources

$ ruff check launch/launch/actions/execute_process.py launch_yaml/test/launch_yaml/test_executable.py
All checks passed!

$ python direct_parse_respawn_max_retries.py  # patched worktree
"'0'" -> 0 (int)
"''" -> ValueError: Attribute respawn_max_retries of Entity `executable` expected to be an integer but got ``
'true' -> ValueError: Attribute respawn_max_retries of Entity `executable` expected to be an integer but got `True`

$ python -m pytest -q launch_yaml/test/launch_yaml/test_executable.py::test_executable_respawn_max_retries_string_zero launch_yaml/test/launch_yaml/test_executable.py::test_executable_respawn_max_retries_empty_string_error launch_yaml/test/launch_yaml/test_executable.py::test_executable_respawn_max_retries_bool_error
...                                                                      [100%]
=============================== warnings summary ===============================
test/launch_yaml/test_executable.py::test_executable_respawn_max_retries_string_zero
  $WORKSPACE/ros2-contrib/worktrees/launch-968/launch/launch/frontend/parser.py:96: DeprecationWarning: SelectableGroups dict interface is deprecated. Use select.
    groups = entry_points.get('launch.frontend.parser', [])

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
3 passed, 1 warning in 0.11s

Test plan

• git diff --check
• python3 -m py_compile launch/launch/actions/execute_process.py launch_yaml/test/launch_yaml/test_executable.py
• ruff check launch/launch/actions/execute_process.py launch_yaml/test/launch_yaml/test_executable.py
• Direct parser check for '0', '', and true
• python -m pytest -q launch_yaml/test/launch_yaml/test_executable.py::test_executable_respawn_max_retries_string_zero launch_yaml/test/launch_yaml/test_executable.py::test_executable_respawn_max_retries_empty_string_error launch_yaml/test/launch_yaml/test_executable.py::test_executable_respawn_max_retries_bool_error