feat: add preflight config validation for active workflows

[lipikaramaswamy]

Summary

This PR adds a public Anonymizer.validate_config() method to preflight-check active workflow configuration against the parsed model pool. We reuse the same preflight path in run() and preview() so invalid model selections fail before workflow execution.

We validate only the aliases required for the active config:

• detection aliases are always validated, as they are required for all workflows
• substitute alias is checked only when Substitute is selected in replace mode.
• rewrite aliases are checked only in rewrite mode

We also add InvalidConfigError so public config validation failures surface consistently.

Test Plan

• uv run pytest tests/interface/test_anonymizer_interface.py tests/engine/test_model_loader.py
• Optional full suite after unrelated payload work is resolved: uv run --group dev pytest

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Refactoring

Testing

• Extended model-loader and interface tests, and moved shared validation fixtures into tests/conftest.py; all tests pass locally
• Notebooks run end-to-end

Related Issues

Closes #5

[andreatgretel]

Looks good overall. I left two small nits around config validation, but nothing blocking from my side.