tests: add DemoConfig test helper and migrate tests to strict BaseConfig

[tomdurrant]

Summary:

• Add tests/test_helpers.py DemoConfig to allow tests to pass arg1/arg2 without relaxing production BaseConfig.
• Migrate tests that previously constructed BaseConfig(arg1=..., arg2=...) to use DemoConfig.
• Restore BaseConfig to a single noop call and keep production models strict (extra='forbid').

Notes:

• DemoConfig is test-only and intentionally not registered in production unions/discriminators.
• I did not change ModelRun unions or production registration of config types.

Please review and merge when ready.