Handle invalid structure YAML in generate without tracebacks or partial side effects #164

Summary

structkit generate can raise raw tracebacks when the structure definition is invalid YAML or when the YAML top-level value is not a mapping. It also creates the output directory before fully validating the structure config, which can leave partial side effects after a config-load failure.

Evidence

Observed locally with the current code:

python -m structkit.main generate --non-interactive invalid.yaml out
Generating structure
  Structure definition: /tmp/.../invalid.yaml
  Base path: /tmp/.../out
Creating base path: /tmp/.../out
Traceback ...
yaml.parser.ParserError: while parsing a flow node

For a YAML file whose top-level value is a list:

python -m structkit.main generate --non-interactive list.yaml out
Traceback ...
AttributeError: 'list' object has no attribute 'get'

Relevant code:

• structkit/commands/generate.py:_load_yaml_config does not catch yaml.YAMLError/OSError.
• GenerateCommand.execute assumes the loaded config has .get(...).
• execute creates base_path before loading/validating the config.

Proposed fix

• Make _load_yaml_config return a structured clean failure or raise a domain error for invalid YAML/read errors.
• Validate that loaded config is a mapping before accessing .get.
• Load and validate the structure config before creating base_path where practical.
• Convert expected config-load failures into ❗ ... messages and exit code 1.

Acceptance criteria

• Invalid YAML in a structure exits 1 with no Traceback.
• Non-mapping top-level YAML exits 1 with a clear message such as top-level YAML content must be a mapping.
• Missing local structure files have clean errors.
• Generation does not create the output directory when config loading/shape validation fails.
• Tests cover invalid YAML, top-level list YAML, and missing file cases.