Improve Python custom validator example

[aryanputta]

Summary

• updates the Python example custom validator module so it demonstrates a real validation failure path
• wires the example YAML to the named damping-ratio validator
• documents how Python examples pass a custom validation module through generate_parameter_module

Fixes #321.

Validation

• python3 -m py_compile example_python/generate_parameter_module_example/custom_validation.py
• direct validator behavior checks for valid, wrong-size, non-finite, non-positive, and out-of-bounds inputs
• generated the Python module from example_python/generate_parameter_module_example/parameters.yaml and parsed it with ast.parse
• git diff --check
• env SKIP=ament_copyright pre-commit run --files README.md example_python/generate_parameter_module_example/custom_validation.py example_python/generate_parameter_module_example/parameters.yaml

ament_copyright is not installed in my local environment, so I could not run that hook locally. All other touched-file pre-commit hooks passed.

[christophfroehlich]

Thank you for improving this.

Please use the same logic as in the c++ example, or adapt both. I try to have the same content in both examples. Can you also leave a review in #361 for that reason please?